ELECTRICAL AND MECHANICAL CHARACTERISTICS OF THE ATRIUM OF THE WHELK BUSYCON CANALICULATUM

1. The mean resting potential of 22 atrial preparations of Busycon hea rt was 42.5 mV, examined by the sucrose gap technique. Spontaneous act ion potentials of 8 - 18 mV amplitude occurred in repeated cycles of b urst activity, generating burst patterned phasic contractile activity. 2. Isolated ventricles showed slow (1 - 3 beats min(-1)) constant myo genic contractile activity, suggesting that the primary driving pacema ker map reside in the atrium. 3. The atrial electrocardiogram commence d with a small prepotential leading to a plateau like phase and termin ated with a sharp spike potential. 4. Acetylcholine (ACh) at high conc entrations depolarised the atrium by 5 - 8 mV and induced strong tonic contractures while suppressing spontaneous action potentials, suggest ing an overall inhibitory role in downregulating cardiac intrinsic myo genic rhythms. 5. Serotonin (5-hydroxytryptamine, 5HT) was consistentl y excitatory, enhancing both action potential amplitude and rhythmic c ontractions by up to 50% at concentrations of 5 x 10(-7) to 10(-5) M. Neither methysergide nor metoclopramide affected atrial responses to 5 HT and the 5HT(1) antagonist metitipine simply increased action potent ial discharge in the rhythmic cycle. The vertebrate 5HT(1-3), receptor classification is inappropriate to this molluscan preparation.6. The atrium was very sensitive to the tetrapeptides FMRF- and FLRFamide, bu t the enhanced phasic contractions were not accompanied by alteration of resting potential or action potential amplitude, suggestive of neur omodulatory upregulation involving a secondary messenger. The related peptide SCP-B was without effect on the preparation, but GAPFLRFamide was excitatory, although much less so than FMRF- and FLRFamide. 7. Nei ther adenosine and ATP nor guanosine and GTP affected intrinsic atrial electrical or mechanical activity, suggesting that there was no nonch olinergic, nonaminergic element to cardiac neuro modulation in this sp ecies. Only ACh, 5HT and FMRF/FLRFamide could be assigned clear roles in this respect. Copyright (C) 1996 Elsevier Science Inc.