Intracellular studies of the electrophysiological properties of cultured intracardiac neurones of the guinea-pig.

1. The electrophysiological properties of intracardiac neurones cultured from ganglia within the atria and interatrial septum of the new-born guinea-pig heart were studied using intracellular micro-electrodes. 2. Three types of neurones with resting membrane potentials in the range -45 to -76 mV were detected. The first type, AHs cells, had high (15-28 mV) firing thresholds, pronounced slow post-spike after-hyperpolarizations and fired only once to prolonged intrasomal current injection. The second type, AHm cells, were similar to AHs cells, except that they could fire short bursts of spikes (100-400 ms) at the onset of current injection. The third type, M cells, had low firing thresholds (10-15 mV), no slow after-hyperpolarizations and produced non-adapting trains of action potentials in response to depolarizing current injection. 3. The generation of action potentials in M cells was prevented by tetrodotoxin (TTX; 0.3 microM), whereas in AHs and AHm cells action potentials displayed a channel blockade using solutions containing the divalent cations cadmium, cobalt or manganese (0.02-1 mM). 4. The post-spike after-hyperpolarization in AHs and AHm cells was abolished by the removal of extracellular calcium, shortened in solutions containing the calcium entry blockers CdCl2, MnSO4 and CoCl2 (0.02-1 mM) and prolonged by the addition of calcium (5.0 mM), tetraethylammonium (1-3 mM), 4-aminopyridine (1-3 mM), cyanide (10 microM) or caffeine (100 microM) to the perfusate. 5. The reversal potential of the post-spike after-hyperpolarization was -89.1 mV. This value changed by 62.9 mV for a 10-fold increase in extracellular potassium concentration. 6. The peak conductance change during the post-spike after-hyperpolarization (gK,Ca), was largely independent of membrane potential between -50 and -110 mV. The peak increase in gK,Ca and the duration of the after-hyperpolarization increased with the number of spikes preceding it. 7. It is concluded that calcium entry during the action potential is responsible for the activation of an outward potassium current in the two types of AH cells; the roles played by intracellular calcium extrusion as well as sequestration mechanisms in the generation of the response are discussed.