Effect of calcium on the spontaneous activities of the oyster myocardium in sodium free solution

Electrophysiology of cardiac myocytes of Aplysia brasiliana

The electrical properties of Aplysia brasiliana myogenic heart were evaluated. Two distinct types of action potentials (APs) were recorded from intact hearts, an AP with a slow rising phase followed by a slow repolarizing phase and an AP with a 'fast' depolarizing phase followed by a plateau. Although these two APs differ in their rates of depolarization (2.2 x 0.3 V/s), both APs were abolished by the addition of Co2+, Mn2+ and nifedipine or by omitting Ca2+ from the external solution. These data suggest that a Ca2+ inward current is responsible for the generation of both types of APs. Two outward currents activated at -40 mV membrane potential were prominent in isolated cardiac myocytes: a fast activating, fast inactivating outward current similar to the A-type K+ current and a slow activating outward current with kinetics similar to the delayed rectifier K+ current were recorded under voltage clamp conditions. Based on the effects of 4-AP and TEA on the electrical properties of ventricular myocytes, we suggest that the fast kinetic outward current substantially attenuates the peak values of the APs and that the slow activating outward current is involved on membrane repolarization.

The ionic basis of cardiac activity in the bivalve mollusc Perna perna

The ionic basis of cardiac activity and aspects of excitation-contraction (E-C) coupling were investigated in the isolated heart of the bivalve mollusc Perna perna, using the sucrose-gap technique. The role of the principal ions was established employing artificial seawater, in which specific ion concentrations were modified, and ion channel blockers. The mean membrane resting potential (MP) and the action potential (AP) were -33+/-0.7 mV (n=89) and 13+/-0.3 mV (n=71), respectively. The MP potential was primarily dependent on K(+) ions. Three types of cardiac APs were identified: fast, slow and spike-plateau potentials. Cardiac activity was maintained in Na(+)- or Ca(2+)-free salines but ceased when either Cd(2+) or EDTA was added to these salines. Other Ca(2+) channel blockers reduced the amplitude and increased duration of the cardiac APs. Tetrodotoxin (TTX) and procaine did not alter the AP. The data showed that the depolarizing phase of the AP was dependent on Ca(2+) influx while the plateau phase, when present, resulted from Na(+) influx that was modulated by Ca(2+). The mechanical responses were more sensitive to changes in extracellular Ca(2+) concentration than were the electrical responses.

Modulatory mechanisms in the isolated internally perfused ventricle of the whelk Busycon canaliculatum

1. Isolated cannulated ventricles commenced spontaneous beating on application of perfusion pressure of 10 cm water. Complete hearts showed a fast patterned cyclical rhythm, whereas ventricles devoid of atrial material showed a continuous slow rhythm. 2. Perfused ventricles were inhibited by ACh with a threshold at 10(-8) mol l-1 and arrested at 10(-7) mol l-1, and ventricles under stimulation by 5HT could be arrested by ACh at this concentration. 3. Perfused ventricles were stimulated by 5HT, with threshold at 10(-9) mol l-1 and maximum at 10(-5) mol l-1. Metoclopramide was without affect on 5HT responses, but metitipine and methysergide did inhibit such responses suggesting that the 5HT receptor present possessed mixed properties of the vertebrate 5-HT1 and 5-HT2 receptor subtypes. 4. Ventricles were very sensitive to the excitatory actions of FMRFamide in the 10(-9) to 10(-5) mol l-1 range. Preparations were insensitive to GAPFLRFamide, but SCP-B was modestly excitatory (threshold 10(-7) mol l-1). 5. Preparations were not significantly affected by adenosine, ATP, and guanosine, but GTP was strongly excitatory at 10(-7) mol l-1. 6. 5HT and FMRFamide responses were additive. Preparations responded strongly to the adenylate cyclase activator forskolin and dibutyryl cAMP enhanced spontaneous contractions and 5HT responses, suggesting that the 5HT receptor may operate via a cAMP secondary mechanism. 7. The IP3 inhibitor lithium (10 mmol l-1), caused slight inhibition of FMRFamide responses, suggesting that the receptor to this peptide may operate via IP3 as a second messenger. 8. Neuromodulation in this preparation would appear to involve ACh as inhibitor, 5HT and FMRFamide as upregulators, with no clear roles for FMRFamide-related peptides and GTP.

Ionic dependency of membrane potential and autorhythmicity in the atrium of the whelk Busycon canaliculatum

1. Calcium-free media usually caused a cessation of all electrical and mechanical activity of the Busycon atrium. Where any electrical activity survived, the action potential consisted of a pre- and plateau-like potential devoid of the usual terminal spike. 2. High Ca salines induced tonic force, membrane depolarization and reduction in generation of spontaneous action potentials. The Ca ionophore A23187 enhanced contractions and the SR CaATPase inhibitor cyclopiazonic acid induced slight depolarization, tonic contractures and increased action potential firing. 3. The inorganic Ca antagonist Co2+ was without effect on the preparations, although the lanthanide Gd3+ inhibited contractions and spontaneous action potentials as well as inducing membrane potential depolarization. 4. The organic Ca entry-blocker nifedipine enhanced both spontaneous action potential amplitude and the phasic contractions they generated. 5. High K salines considerably depolarized atrial preparations with accompanying large tonic contractures and suppression of action potentials. The K channel-blocker 4AP enhanced action potential amplitude with slight increase in contractions, and TEA depolarized the atrium, and enhanced action potentials and rhythmic contractions. 6. Sodium-free salines strongly hyperpolarized atrial preparations and abolished spontaneous action potentials and, on washout, the membrane potential became temporarily unstable. In 2 preparations, low chloride and chloride-free media induced significant membrane potential hyperpolarization. 7. It is concluded that, in the atrium, the resting membrane potential is largely determined by the transmembrane K gradient, but with significant conductances to Na and Cl though probably not Ca. The action potential spike appears to be a Ca-dependent event and the plateau-like phase may be a Na-dependent event.

Three types of acetylcholine response in bivalve heart muscle cells

1. The acetylcholine (ACh) responses of cardiac muscle cells from three species of bivalves were studied by intracellular recording and ACh ionophoresis. Heart muscle contraction was abolished by bathing in artificial sea water in which Mn(2+) had been substituted for Ca(2+).2. Three different types of membrane potential changes were observed in response to ACh pulses: a slow hyperpolarization in the clam Mercenaria mercenaria, a rapid depolarization which was sometimes followed by a slower hyperpolarization in the muscle Mytilus edulis, and a biphasic response consisting of both a rapid depolarization and slower hyperpolarization in the oyster Crassostrea virginica. All responses were accompanied by an increase in membrane conductance, as measured by passing constant current pulses with an extracellular suction electrode.3. The hyperpolarizing response in all three species was blocked most effectively by methylxylocholine and not very effectively by tubocurarine or hexamethonium. The depolarizing response in Mytilus was blocked preferentially by tubocurarine and hexamethonium, while the depolarizing response in Crassostrea was blocked effectively by tubocurarine only.4. The hyperpolarizing response in all three species was not altered by Cl(-)-free or Na(+)-free sea water, but was affected by changes in external K(+). The rate of change of the inversion potential of this response with change in [K(+)](o) was 59 mV per tenfold concentration change. The depolarizing response of Mytilus was not altered by changes in external Cl(-) or K(+), but was abolished in Na(+)-free sea water. The depolarizing response of Crassostrea was insensitive to external Na(+) or K(+), inverted at -31 mV in normal sea-water, and was altered by changes in [Cl(-)](o). Substituting for Cl(-) with sulphate shifted the inversion potential at a rate of 56-61 mV per tenfold Cl(-) concentration change.5. The three types of ACh response seen in bivalve heart muscle are similar with respect to time course, pharmacological sensitivity and ionic mechanism to the three types of ACh response described in Aplysia central neurones. Analogies can also be drawn with vertebrate ACh responses.