DIHYDROPYRIDINE-SENSITIVE VOLTAGE-DEPENDENT CALCIUM-CHANNEL IN THE SARCOLEMMAL MEMBRANE OF CRUSTACEAN MUSCLE

Single-channel currents through calcium channels in muscle of a marine crustacean, the isopod Idotea baltica, were investigated in cell-atta ched patches. Inward barium currents were strongly voltage-dependent, and the channels were closed at the cell's resting membrane potential. The open probability (P-o) increased e-fold for an 8.2 mV (+/-2.4, n = 13) depolarization. Channel openings were mainly brief (<0.3 ms) and evenly distributed throughout 100-ms pulses. Averaged, quasimacroscop ic currents showed fast activation and deactivation and did not inacti vate during 100-ms test pulses. Similarly, channel activity persisted at steadily depolarized holding potentials. With 200 mM Ba2+ as charge carrier, the average slope conductance from the unitary currents betw een +30 and +80 mV, was 20 pS (+/-2.6, n = 12). The proportion of long openings, which were very infrequent under control conditions, was gr eatly increased by preincubation of the muscle fibers with the calcium channel agonist, the dihydropyridine Bay K8644 (10-100 mu M). Propert ies of these currents resemble those through the L-type calcium channe ls of mammalian nerve, smooth muscle, and cardiac muscle cells.