Monoamines and the isolated auricle of Sepia officinalis: Are there beta-like receptors in the heart of a cephalopod?

Pharmacological examinations of isolated auricles from Sepia officinalis we re carried out to analyze the putative role of the monoaminergic transmitte r/receptor system in the control of auricle function. In conjunction with h istofluorescence studies and HPLC analyses, evidence of a double excitatory serotonergic and noradrenergic innervation of the auricles was obtained. S erotonin-induced positive chronotropic and inotropic effects were blocked b y mianserin (5-HT1 and 5-HT2) but not by cyproheptadine (5-HT2), It is assu med that the auricular serotonin (5-HT) receptor represents a 5-HT1-like su btype and is not identical to the ventricular 5-HT receptor. Noradrenaline, adrenaline and dopamine evoked mainly positive chronotropic reactions and less prominent positive inotropic reactions. The potency range (pD(2) frequ ency: noradrenaline 6.65 much greater than adrenaline 5.69 > dopamine 5.34; pot amplitude: noradrenaline 6.09 greater than or equal to adrenaline 5.91 > dopamine 5.33) indicates out that noradrenaline might be the effective n eurotransmitter in vivo. The alpha-mimetics clonidine (alpha(2)) and phenyl ephrine (alpha(1)) induced positive chronotropic and inotropic effects, whi le the beta-mimetics albuterol (beta(2)>beta(1)) and dobutamine (beta(1)) r evealed only positive inotropic reactions. The beta-agonist isoprenaline mi micked the positive chronotropic effects of noradrenaline and induced the s trongest positive inotropic effects of all the agonists tested. Urapidil (a lpha(1)) or phentolamine (alpha(1) and alpha(2)) blocked only the positive chronotropic effects of noradrenaline and isoprenaline, The positive inotro pic effects of isoprenaline could be blocked by the adenylate cyclase inhib itors MDL-12,330A or SQ-22,536, which had no effect on the chronotropic eff ects of isoprenaline. These results suggest that two catecholaminergic rece ptors are present in the auricles of Sepia officinalis: an alpha-like adren oreceptor mediating mainly chronotropic effects, and a beta-like receptor w hich appears to mediate inotropic effects by activating the cyclic AMP path way. These results suggest that the auricles exert a regulatory effect on v entricular performance.