The effects of adenosine and adenine nucleotides on haemolymph velocity and on heart rate and scaphognathite

ecological demands interfering with oxygen delivery such as environmental hypoxia (Butler et al., 1978; Wheatly and Taylor, 1981; McMahon, 1992; Reiber et al., 1992; Airries and McMahon, 1994), air exposure (Taylor and Wheatly, 1981; Airries and McMahon, 1996) or exercise (Herreid et al., 1983; Hamilton and Houlihan, 1992; Hokkanen and DeMont, 1992; Reiber, 1994; Reiber et al., 1997) with various physiological adaptations. An increase in gill ventilation, heart rate, stroke volume and haemolymph flow and a redistribution of the haemolymph to areas of enhanced oxygen demand may intensify the supply of oxygen to the tissues (Wheatly and Taylor, 1981; McMahon and Burnett, 1990; McMahon, 1995a). Based on a deluge of work (McMahon and Wilkens, 1983), it was suggested (McMahon and Burnett, 1990) that the circulatory system of advanced decapod crustaceans might allow fine graded regulation of cardiac output and haemolymph flow as well as selective haemolymph distribution, and thereby augment oxygen supply to the tissues. To a certain extent, ventilatory and circulatory control mechanisms in advanced crustaceans may even be functionally equivalent to those of the vertebrate autonomic system (McMahon, 1995b). These statements and the availability of two non-invasive methods raised new interest in the efficiency of the crustacean cardiovascular physiology. In particular, the techniques of pulsed-Doppler-flowmetry and of photoplethysmography lend themselves well to the measurement of heart rate, haemolymph velocity (Reiber et al., 1992) and ventilation rate (Depledge, 1984). Amongst cardiovascular adaptations, chronotropic and inotropic effects of the heart have been documented for many crustaceans. The influence of mediators such as proctolin, octopamine, dopamine, serotonine and crustacean cardioactive peptides on the circulatory system has been investigated (Wilkens et al., 1985; McGaw et al., 1994). These neurohormones, which are released from the pericardial organ, pass directly to the heart where they have been found to have both chronotropic and inotropic effects (Cooke and Sullivan, 1982). In addition to hormones, metabolites such as lactate or urate originating from muscle tissue or the hepatopancreas function as metabolic modulators, increasing the oxygen affinity of haemocyanin in many crustaceans (Truchot, 1980; Morris et al., 1985). These internal mediators may transfer to the cardio-ventilatory system information related to external abiotic factors such as lack of ambient oxygen or flight and fight responses. Amongst the many internal factors known to act systemically in animals, adenosine is predominant since it can exert several important cardiovascular actions. In vertebrates especially, the adenosine-induced increase in coronary blood flow has been the subject of numerous investigations (Drury 947 The Journal of Experimental Biology 204, 947–957 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3189