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 recordin gs have revealed six different ionic currents in myocytes freshly dissociat ed from squid cardiac tissues (branchial and systemic hearts). Three types of outward potassium currents were identified: first, a transient outward v oltage-activated A-current (I-A), blocked by 4-aminopyridine, and inactivat ed by holding the cells at a potential of -40 mV; second, an outward, volta ge-activated, delayed rectifier current with a sustained time course (IK); and third, an outward, calcium-dependent, potassium current (I-K(Ca)) sensi tive to Co2+ and apamin, and with the characteristic N-shaped current volta ge relationship. Three inward voltage-activated currents were also identified. First, a rapi dly activating and inactivating, sodium current (I-Na), blocked by tetrodot oxin, inactivated at holding potentials more positive than -40 mV, and abol ished when external sodium was replaced by choline. Second, an L-type calci um current (I-Ca,I-L) With a sustained time course, suppressed by nifedipin e 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 d istinguished from the L-type current by differences in its voltage dependen ce. It also had a more transient time course, was activated at more negativ e potentials, and resembled the previously described low-voltage-activated, T-type calcium current.