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AMER. ZOOL., 35:37^»8 (1995) Regulation of the Cardiovascular System in Crayfish12 JERREL L. WILKENS Department of Biological Sciences, University of Calgary, Calgary, Alberta T2N 1N4, Canada INTRODUCTION The integrated function of the cardiovascular system of a crayfish is required in order to meet the circulatory requirements of the animal. These requirements are likely to change over time and in different situations. The cardiovascular system is made up of a neurogenic heart which pumps blood into an open vascular system made of musclefree arteries. The arteries branch into finer arterioles which in turn discharge into capillary-like lacunae. Blood is collected into a number of sinuses (Greenaway and Farrelly, 1984) and returns to the heart via a series of channel-like extensions of these sinuses. A brief survey of some of the cardiovascular adjustments which have been identified in whole animal studies is presented first to anticipate some of the control points which are important in this system. The mechanisms by which adjustments occur will form the main focus of this paper. Most of the data presented here is derived from studies on crayfish, but those derived from other decapods will also be presented where comparable information is lacking for crayfish. WHOLE ANIMAL OBSERVATIONS At the level of the heart, beat rate (fH), stroke volume (Vs) and cardiac output (0) have received the most attention. Rate is easy to monitor with AC amplifiers and impedence converters while Q has been calculated from applications of the Fick Equation or by a thermodilution technique (Burnett et al., 1981). Stroke volume is obtained as the quotient of Q divided by fH. 1 From the Symposium Physiology and Adaptations in Crayfish presented at the Annual Meeting of the American Society of Zoologists, 26-30 December 1993, at Los Angeles, California. 2 Abbreviations: CA, cardioaccelerator nerves; CCAP, crustacean cardioactive pcptide; CI, cardioinhibitor nerves; DA, dopamine; FaRPs, FLRFamiderelated peptides; fH> heart rate; OA, octopamine; Q» cardiac output; PRO, proctolin; 5-HT, serotonin or 5-hydroxytryptamine; V,, stroke volume. 37 Rapid changes in fH, including cardiac arrest, occur in response to a variety of external stimuli including the startle response (Larimer and Tindel, 1966; Wilkens, 1987). During forced exercise both fH and 0 a r e increased in most cases (Cancer magister [McMahon et ai, 1979], Carcinus maenas [Houlihan et al., 1984; Hamilton and Houlihan, 1992], fH but not Q in Callinectes sapidus [Booth et al., 1982]). Stroke volume increased in a number of cases (McMahon et al., 1979; Depledge, 1978) but decreased in others (Booth et al., 1982). In response to hypoxia, two species of crayfish were able to maintain or increase Q m the face of bradycardia by increasing Vs (Orconectes rusticus [Wilkes and McMahon, 1982], Procambarus clarkii [Reiber et al., 1992]). Thus, there is evidence that crustaceans have the ability to independently regulate fH and Vs. The distribution of blood to the seven arteries (nomenclature in McLaughlin, 1983) which leave the single chambered heart can also be controlled. By employing a noninvasive pulsed-Doppler flowmeter technique, changes in the flow profile among these arteries have been observed in crayfish, crabs and lobsters during hypoxia and in response to neurohormones (Reiber et al., 1992; Airriess and McMahon, 1992; McMahon, 1992; Reiber, 1995). Recent reports confirm that different profiles of distribution do occur in different situations, e.g., rest, locomotion and hypoxia (Airriess and McMahon, 1994; McGaw et al., 1995). OBSERVATIONS FROM ISOLATED SYSTEMS In order to identify and characterize the mechanisms available and/or used in cardiovascular regulation I will take the reductionist approach of describing research based on isolated parts of the system. All of the figures presented in this paper are of unpublished data. 38 JERREL L. WILKENS Wilkens and McMahon, 1994). As outflow resistance is increased, ventricular pressure increases but only to the level produced 40 when all outflow is prevented. V5 decreases eventually to zero while fH is unaltered. In myogenic molluscan (Smith, 1987) and ver20 <0 50 100 150 ZOO 250 tebrate (Berne and Levy, 1992) hearts venous return of blood preloads the heart 1.2 30 during diastole. The heart compensates for increases in preload by increasing Vs and sometimes also by increasing fH. There is 15 c no venous return to crayfish hearts, but rather theyfillfrom the pericardial sinus via E ostia. The end-diastolic volume of the deca0.0 50 100 150 200 250 0 pod ventricle is determined by the amount of elastic recoil of the alary and suspensory ligaments. Thus, the stretch imposed by the ^ 1.2 ligaments may be viewed as imparting an o 0_ external form of preload. It is not known •^ 0.B whether these external suspensory systems c o are able to adjust end-distolic volume, but a." 0.4 in crayfish the pericardial septum which is 0 50 100 150 200 250 attached to the ventral wall of the heart by numerous connective tissue strands is Outflow resistance (Pa/ml.min ) invested with muscle bands and appears to FIG. 1. In situ Procambarus clarkii hearts respond expand the ventral wall of the heart during passively to increases in outflow resistance. Heart rate, diastole (George et al., 1955). In C. magister as determined by the burst rate of the cardiac ganglion, the origins of the alary ligaments contain is unaffected; stroke volume decreases as resistance increases and the ventricular systolic pressure increases slips of striated muscle (Volk, 1988). The toward the level which occurs during isovolumic con- effects on cardiac output of altering the tentractions when all outflow is prevented. Mean ± SEM, sion in either of these ligamentous systems n = 5 (Wilkens and Walker, unpublished). is not known. Artificially increasing preload by direct heart perfusion in crayfish does stretch the heart wall and does cause Semi-isolated heart increases in systolic pressure and Vs, but Crayfish hearts are neurogenic in that the heart rate is either unaffected or slowed basic heart beat and rhythm are determined (Wilkens, 1993). It must be emphasized that by the nervous output of the cardiac gan- this direct form of preload will never occur glion which is located on the inner dorsal in an intact animal. Since no automatic wall of the heart (Alexandrowicz, 1932). The compensatory mechanisms are known, we myocardium itself is not spontaneously must look for control mechanisms external active. Semi-isolated hearts, "on the half to the heart for the causes of changes in fH shell," suspended by their suspensory liga- and Vs. ments, beat and pump fluid with a strong and regular rhythm for hours (Wilkens and The extrinsic control of fH can arise from Walker, 1992; Wilkens and Mercier, 1993). the cardioregulatory nervous and neuroThe proximal portions of all arteries and all hormonal inputs. The factors which could extrinsic innervation to the heart remain account for the changes in Vs in intact aniintact and each is accessible for manipula- mals are not known, but candidates include tion. (i) changes in end-distolic volume resulting /. Lack of autoregulation. —Isolated from dynamic changes in alary ligament hearts do not show compensatory responses tension (unstudied), (ii) changes in systolic to increases in arterial afterload or to ejection fraction which might accompany increases in peripheral resistance (Fig. 1, also neurohormone induced increases in con60 39 CARDIOVASCULAR REGULATION 70 120 r r (bpm 60 50 <u 40 L L o 100 80 3 Heart cr 50 IALJ. 2 7 x/' ° Ligament loss • Oenervotion en -*l 40 i V 1 o Control 60 c T -4 30 'E E 20 0 10 20 30 40 10 20 30 40 1.0 0.8 0.6 0.4 Stimulating frequency (Hz) FIG. 2. The cardioacceleratory nerves exert only chronotropic effects on heart performance. The increases in cardiac output of in situ Procambarus clarkii hearts results from the increased fH, while ventricular systolic pressure and stroke volume remain relatively constant over the same range of stimulating frequencies. Experiments performed at 18°C in crayfish saline. Mean ± SEM, n = 5 (Wilkens and Walker, unpublished). 40 -10 10 20 30 40 Time (min) FIG. 3. Effects of treadmill locomotion on heart rate in control Homarus americanus, and after first reducing cardiac output by severing the two dorso-anterior alary ligaments and then after severing the two dorsal nerves. Following each surgical procedure the lobsters were allowed 24 hr to recover before being tested again. The sea water in the underwater treadmill was continuously exchanged with the holding aquarium and was maintained at 12°C. Mean ± SEM, error bars for controls not shown, but they do not overlap those of the other traces except at the end of the 30 min period. Treadmill speed was 1.7 m • min -' (Guirguis and Wilkens, 1995). frequency of the inhibitors above 12 Hz causes cardiac arrest and there is strong reciprocity between activity in the accelerators and the inhibitors. We have recently been examining the effects of regulatory nerve input on the pumping performance of isotractility (Pvent, see below) and/or (iii) chan- lated hearts (Wilkens and Walker, 1992). ges in total peripheral resistance (see below). During CA stimulation fH and Q increase, 2. Nervous control.—The cardioregula- but Vs and PvenI do not change (Fig. 2). Durtory innervation of the heart consists of a ing CI stimulation heart rate decreases linpair of bilateral dorsal nerves, each of which early with increasing frequency of stimucarries two cardioaccelerator (CA) and one lation, but Vs falls only at stimulation rates inhibitor (CI) axons. These nerves arise from above 10 Hz (data not shown). Thus, the the subesophageal ganglion (Wiersma and regulatory nerves provide an effective means Novitski, 1942; Maynard, 1953; Florey, of making rapid adjustments in fH and Q, 1960; Field and Larimer, 1975). The CIs while the increases in Vs, as observed in synapse only on the cardiac ganglion, while intact animals, probably do not arise from the CAs may synapse on both the cardiac the effects of cardioregulatory nervous ganglion and sometimes directly on the inputs. myocardium as well (Maynard, 1960; The importance of the cardioregulatory Yazawa and Kuwasawa, 1984). In situ, both nerves on the overall control of fH is illusaxon types exhibit a low background level trated by the responses of lobsters {Homaof spike activity which is punctuated by high rus americanus) during treadmill locomofrequency bursts of spikes (Field and Lari- tion (Fig. 3). In controls, fH increases from mer, 1975; Young, 1978). Increases in the rest rates to a plateau in the first two minutes 40 JERREL L. WILKENS 100 r Procambarus 80 60 40 20 o DA 5-HT OA c O a. 50 PR OF, CCAP f Homarus 40 30 20 10 <o DA 5-HT OA PR CCAP PR 5-HT CCAP FIG. 4. Comparisons of the effects of pericardial organ neurohormones on heart rate, expressed as percent change from control, in three different decapod crustaceans. The semi-isolated hearts were continuously perfused at 1 ml • min~', 2 ml • min~' in Homarus, with an appropriate saline. Each neurohormone was applied for 30 sec by switching the perfusion pump supply from control saline to saline containing the appropriate compound. Procambarus: All compounds except DF2 were tested at 0.1 /tin, DF2 was tested at 10 nm (Guirguis and Wilkens, unpublished). Homarus (Kuramoto, Wilkens, and McMahon, unpublished) and Carcinus (Saver and Wilkens, unpublished): DA, OA and 5-HT were tested at 10 nm, the peptides were tested at 10 nm. Mean ± SEM, n = 3-9. after the onset of walking and is maintained at relatively constant levels for the duration of a 30 minute walk. After the cardiac output is reduced by about 25 percent by cutting two of the alary ligaments (Wilkens and McMahon, 1992), fH is elevated both at rest and during locomotion. It seems clear that the animals sense the cardiac insufficiency and compensate by increasing fH. Next, after bilateral dorsal nerve section, resting fH is reduced to the level shown by controls. The reduced resting rates, relative to those during the immediately preceding period when the ligaments were cut, may arise from the removal of tonic CA drive. During walking following combined alary ligament and nerve section, heart rate does increase slowly over about 25 minutes, but does not reach the previous levels seen after ligament cutting. The stimuli responsible for this gradual onset oftachycardia are not known, but they may include neurohormones. Despite this slow increase in heart rate, walking endurance is severely impaired in these animals. 3. Neurohormonal control.—The crayfish heart is quite responsive to the neurohormones known to be released from the pericardial organs and one similar to a peptide found in the hindgut nerve plexes. The actions of most of these hormones are compared on the semi-isolated hearts of crayfish (P. clarkii), crabs (C. maenas) and lobsters (Homarus americanus) in Figure 4. The pericardial organ aminergic neurohormones are 5-hydroxytryptamine (5-HT, serotonin), dopamine (DA) and octopamine (OA) (Cooke and Sullivan, 1982). In the three genera illustrated in Figure 4, 5-HT consistently produced the greatest effect, while DA and OA were less potent. In two other species of crayfish 5-HT, DA and OA each caused increases in the frequency and amplitude of the heart beat (Astacus leptodactylus and Eriphia spinifrons [Florey and Rathmayer, 1978]). All of the pericardial organ peptide neurohormones, crustacean cardioactive peptide (CCAP [Strangier et ai, 1987]), proctolin (PR [Sullivan, 1979]) and several FLRFamide-related peptides (FaRPs, references follow), caused fH to increase (Fig. 4), but see Figure 8 for exceptions. There is some species specificity in the responsiveness to some of these hormones. Crustacean cardioactive peptide produces moderate to strong positive chronotropic and inotropic effects on Orconectes limosus (Strangier and Keller, 1992), P. clarkii and C. maenas (Wilkens and McMahon, 1992; Wilkens and Mercier, 1993). Lobster hearts and those of another crab, C. magister (data for latter not shown) are unresponsive to CCAP. All of 41 CARDIOVASCULAR REGULATION these peptides, except proctolin, cause parallel increase in beat rate and contractility. In all three genera proctolin is weakly chronotropic but strongly inotropic, an indication that this peptide is effective both at the level of the cardiac ganglion (rate effects) and on the myocardium (force effects) (Wilkens and Mercier, 1993, for force data). A number of FMRFamide-related peptides (FaRPs) have been identified in the pericardial organs of crayfish (DF2, DRNFLRFamide and NF1, NRNFLRFamide [Mercier et al., 1993]) and lobsters (Fl, TNRNFLRFamide and F2, SDRNFLRFamide [Trimmer et al., 1987]). In P. clarkii hearts, Fl and F2 increased both heart rate and contraction amplitude (Mercier et al., 1991; Mercier and Russenes, 1992) whereas two FaRPs of insect origin with significantly different amino-terminal extensions (leucomyosuppression, pQDVDHVFLRFamide [Holman et al., 1986], and SchistoFLRFamide, PDVDHVFLRFamide [Robb et al., 1989]) strongly inhibit heart rate while increasing the amplitude of contraction (Fig. 5; also Mercier and Russenes, 1992). The low threshold for the inhibitory peptide SchistoFLRFamide (1 nm) suggested to Mercier that crayfish hearts possess receptors for this or a closely related peptide. A peptide from the crayfish hindgut nerve plexus is identical to locust SchistoFLRFamide at the first six C-terminal residues (Mercier, TeBrugge and Orchard, personal communication). Guirguis and Wilkens found that the inhibitory effects of this peptide on crayfish hearts were long lasting and generally irreversible when tested at l/xm. When the effects of six members of the FaRP family (Fl, F2, DF2, NF1, FLRFamide and FMRFamide) are compared on isolated P. clarkii hearts, the native peptides (DF2 and NF1) are found to be more stimulatory than the lobster peptides (Fl and F2) or the root peptides FLRFamide or FMRFamide (Price and Greenberg, 1977) (Figs. 6, 7). In 3 of 5 hearts, the lowest concentrations of DF2 (Fig. 8), NF1, FLRFand FMRFamide caused biphasic responses, bradycardia and depressed systolic pres- Rate Pressure o -11 -10 -9 -8 log [F21 100n Rate Pressure 60 a o> S 5 20 -20 -60-100 -9 -8 -7 log [Schisto] FIG. 5. Concentration-response relationships comparing the chronotropic and inotropic effects of the peptides F2 and SchistoFLRFamide on semi isolated Procambarus clarkii hearts. Plotted are the relative changes in heart rate after 1 min and arterial pressure after 3 min during a 5 min period of perfusion with the neurohormone. These observations were made during June, 1990, mean ± SEM, n = 3-8 at each concentration (Mercier and Wilkens, unpublished). sure, followed by tachycardia and increased systolic pressure, while high concentrations only caused tachycardia and increased pressure. On the other two hearts all concentrations caused dose-dependent increases in all parameters; however, on one of these hearts a 0.01 nm challenge produced inhibition similar to that shown for 0.1 nm in Figure 6. It appears as if the heart may contain high affinity receptors which cause bradycardia and low affinity ones responsible for tachycardia. The threshold for bradycardia may vary from animal to animal. The inhibitory effects of Fl and F2 were not reported in the study by Mercier and Russenes (1992). Vascular system 1. Cardioarterial valves.— Some of the changes in blood distribution in intact animals documented by the pulsed-Doppler technique may be controlled by the semi- 42 JERREL L. WILKENS 200 NF1 150 100 c (D O -t-> 50 0 200 200 150 150 100 100 50 50 0 0 F2 O -M O 200 CD 200 cn c o 150 150 100 100 50 50 0 0 o FMRF FLRF Concentration (M) FIG. 6. Comparison of the relative chronotropic effects of FaRPs (FMRFamide-related peptides) on in situ Procambarus clarkii hearts. Each of the 5 hearts tested was exposed to all three concentrations of each of the six peptides. The six peptides were first applied at the lowest concentration, then each at the intermediate and finally at the highest concentration; however, the order of presentation among the six was randomized. Each compound was applied at the rate of 1 ml-min-' for 30 sec during continuous perfusion. In three hearts the lower concentrations caused biphasic responses, bradycardia followed by tachycardia. Only the acceleratory responses are plotted here. These data were collected September to December, 1993. Mean ± SEM, n = 5 (Guirguis and Wilkens, unpublished). lunar cardioarterial flap valves located at the origin of each artery at the heart. The valves in crayfish, crabs and lobsters are invested with muscle and are innervated (Alexandrowicz, 1932). In isopods and lobsters they receive excitatory and/or inhibitory innervation (Kihara etai, 1985; Kuramoto et al., 1992). Contraction of valve muscles will stiffen the valve and decrease blood flow through it, while inhibitory input will relax the muscles and allow increased blood flow (Kuramoto and Ebara, 1989; Fujiwara-Tsukamoto et al, 1992). These valves can thus serve the dual functions of passively preventing back flow of blood during diastole and of actively regulating the distribution of blood among the various arteries. The effect of such control is illustrated in the isopod, Bathynomus doederleini, where inhibitory nerve input to the 5th lateral artery valves which control blood supply to the swimmerettes coincides with activation of the swimmerettes (FujiwaraTsukamoto et al, 1992). 43 CARDIOVASCULAR REGULATION NF1 c CD O l_ CD Q_ F2 CD i_ CO CO CD L_ Q. c CD CT> FMRF C O .r: O 20 10 Concentration (M) FIG. 7. Comparison of the relative inotropic effects of six FaRPs on in situ Procambrus clarkii hearts. These data were collected from the same hearts as those shown in Figure 6. In Panulirns japonicus the cardioarterial valves are also excited or inhibited by different cardioactive neurohormones (Kuramoto and Ebara, 1984). Proctolin causes depolarization of the anterior and posterior cardioarterial valves, 5-HT causes hyperpolarization and relaxation of the same valves as well as increases in heart rate, while OA causes contraction of the posterior valve and relaxation of the anterior valve. Thus, each neurohormone should produce a different blood flow distribution pattern. In H. americantis proctolin, 5-HT and F2 cause the sternal arterial valve to contract while dopamine relaxes it (Kuramoto et al, 1992; Kuramoto, Wilkens, and McMahon, 1995). Neurohormones may be important in con- trolling arterial valve tonus, while the nervous control could function over shorter time periods. Crayfish cardioarterial valves have not been studied, but it is anticipated that individual valves will also show differential responsiveness to innervation and to neurohormones. The nervous and neurohormonal control of the cardioarterial valves may be one mechanism of controlling blood distribution. 2. Arteries.—In general, arteries do not contain muscle layers and it has been assumed that they only serve as passive conduits for blood distribution; however, recent evidence shows that they are not passive. In crayfish muscular flap valves are located 44 JERREL L. WILKENS DF 2 10 10 mol L 8 mol L 12 -6 10 -1 mol L 2 min o FIG. 8. Records, typical of 3 of 5 preparations, showing the effects of 30 sec exposures of a perfused Procambarus clarkii heart to three concentrations of the peptide DF2. At 0.1 nm the peptide reduced fH, Pvent and Q, at 10 nm the early inhibitory phase was followed by acceleration and increased contractility, while at 1 nm the peptide caused increases in all three parameters. The non-linear response characteristics of the flow transducer mask to fact that flow increased almost 2-fold during 1 nm DF2 exposure. Note that the increases in fH occur more rapidly than the increases in Pvcm (Guirguis and Wilkens, unpublished). at the origin of the two primary lateral arteries from the dorsal abdominal artery in each abdominal segment and at the bifurcation in the 5th segment. These valve muscles are I 12 'E 10 sensitive to a variety of neural transmitters and neurohormones (Fig. 9). Valve contractions induced by a 60 sec exposure to acetylcholine, and a 30 sec exposure to proctolin and 5-HT relax slowly over about five minutes, whereas it requires more than an hour of washing after a similar 30 sec exposure to Fl or F2 before relaxation begins. These lateral arterial valves are also innervated (Alexandrowicz, 1913) and their physiology is currently under investigation. o CONTROL OF BLOOD PRESSURE AND OF " L U N G " PERFUSION c o -100 100 200 Time (s) 300 400 FIG. 9. The magnitude and time course of response to four neurohormones (5-HT, proctolin, Fl and F2, 30 sec exposure each) and acetylcholine (60 sec exposure) on the resistance of a Procambarus clarkii dorsal abdominal artery during continuous perfusion at 1 mlmin-'. Each compound was presented at 10 nm. To measure the resistance to flow, the artery was cannulated and perfused via a peristaltic pump. The resistance is the quotient of the back pressure generated in the cannula divided by the flow rate (Lovell and Wilkens, unpublished). There is no evidence in any crustacean that the heart or vascular system is involved in the regulation of overall blood pressure. However, in crabs, arrays of dorsoventral muscles span the coelomic space between the dorsal carapace and the flexible anterior lining of the branchial chambers. Contraction or relaxation of these muscles accompanies decreases or increases in blood volume, respectively, and helps restore blood pressure to control levels after such perturbations (C maenas [Rajashekhar and Wilkens, 1991; Taylor and Taylor, 1991; Taylor et ai, 1992], Goniopsis cruentata [Wilkens and Young, 1992]). Contraction of these 45 CARDIOVASCULAR REGULATION 3 x 10-7 M F 2 3 x 10-7 M J FIG. 10. Effects of bath application of two FaRPs on Prcocambarus clarkii hindguts. Each peptide caused an increase in the amplitude of the spontaneous rhythmic contractions of these tissues. The upward deflections represent longitudinal contractions. The decrease in baseline during portions of both traces represent lengthening of the gut strip, probably are a consequence of contractions of the circular muscle layers. Calibrations: verticle = 0.5 nm, horizontal = 2 min (Schmoeckel and Mercier, unpublished). muscles, acting as an auxiliary pump, also facilitates perfusion of the branchiostegal or lung lacunar network. (Wilkens and Young, 1992). It is not known whether crayfish possess homologous dorsoventral muscles; however, perfusion of the branchiostegal lungs may be important to crayfish during air exposure, as when burrowing or migrating over land. A search for such a control system might be fruitful. the act of ingesting food triggers a release of peptides which will facilitate the movement of that food through the gut. Recall also that the FaRPs can increase fH and ventricular pressure. The resulting increased blood flow may be directed toward organs associated with digestive processes. The cardioarterial flap valve at the origin of the dorsal abdominal artery in H. americanus and P. clarkii does not contain muscle and is unresponsive to the FaRPs AN EMERGING STORY OF NEUROHORMONAL (Kuramoto, Wilkens, and McMahon, 1995) INTEGRATION—THE ROLE OF F A R P S IN so that there should be no regulation of DIGESTIVE AND CIRCULATORY blood entry into this large vessel. However, PROCESSES the FaRPs cause powerful and prolonged In P. clarkii the circulating level of closure of the lateral arterial valves which FaRPs is substantially elevated following branch off of the dorsal abdominal artery feeding. FMRFamide-like immunoreac- and contraction of these valves reduces tivity increased 300% one hour after feed- blood flow to the abdominal musculature ing (from 0.16 ± 0.02 nm to 0.63 ± 0.13 and the swimmerettes and causes the presnm, n = 8; Mercier, personal communi- sure in the dorsal abdominal artery to rise. cation). Similar blood levels of FaRPs are A large number of small diameter arteries reported in lobsters, but these levels were branch off of the ventral wall of the dorsal not related to any physiological or behav- abdominal artery to supply blood to the ioral state (Kobierski et al., 1987). FaRPs intestine. The origins of these arteries are also induce strong pyloric and gastric mill not controlled by valves and the FaRPrhythms in the Cancer irroratus and Cancer induced increases in arterial pressure should borealis stomatogastric ganglion (Hooper increase flow of blood to the gut. and Marder, 1984; Weiman et al, 1990) The picture which is emerging is that the and increase motility of isolated crayfish FaRPs released during feeding may help hindgut (Fig. 10). Thus, it would appear that control several systems whose integrated 46 JERREL L. WILKENS functions will aid the processing, digestion and absorption of food. SUMMARY All levels of the cardiovascular system, from the heart to the cardioarterial valves and the dorsal abdominal arterial valves, receive nervous input and all are responsive to neurohormones. The sites of action of neurohormones on the other arteries are not known at present. The challenge for the future is to devise experiments which will allow us to measure which of these control systems are operative to produce the patterns of heart and vascular performance observed in intact animals. ACKNOWLEDGMENTS To participate in this symposium has special significance to me since Milton Fingerman, a co-organizer of the symposium, was my first graduate supervisor at Tulane University 32 years ago. I wish to thank him for being my first real scientific mentor. I also thank Dr. A. J. Mercier for sharing unpublished data and for reviewing the manuscript, and Dr. I. Orchard for samples of the peptides DF2 and NF1. 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