Ionic currents in cardiac myocytes of squid, Alloteuthis subulata.

This paper provides the first study of voltage-sensitive membrane currents present in heart myocytes from cephalopods. Whole cell patch clamp recordings have revealed six different ionic currents in myocytes freshly dissociated from squid cardiac tissues (branchial and systemic hearts). Three types of outward potassium currents were identified: first, a transient outward voltage-activated A-current (IA), blocked by 4-aminopyridine, and inactivated by holding the cells at a potential of -40 mV; second, an outward, voltage-activated, delayed rectifier current with a sustained time course (IK); and third, an outward, calcium-dependent, potassium current (IK(Ca)) sensitive to Co2+ and apamin, and with the characteristic N-shaped current voltage relationship. Three inward voltage-activated currents were also identified. First, a rapidly activating and inactivating, sodium current (INa), blocked by tetrodotoxin, inactivated at holding potentials more positive than -40 mV, and abolished when external sodium was replaced by choline. Second, an L-type calcium current (ICa,L) with a sustained time course, suppressed by nifedipine or Co2+, and enhanced by substituting Ca2+ for Ba2+ in the external medium. The third inward current was also carried by calcium ions, but could be distinguished from the L-type current by differences in its voltage dependence. It also had a more transient time course, was activated at more negative potentials, and resembled the previously described low-voltage-activated, T-type calcium current.