Ionic currents in cardiac myocytes of squid, Alloteuthis subulata

Abstract

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.

Establishment of functional primary cultures of heart cells from the clam Ruditapes decussatus

“…However, I K fast was more sensitive in clam than in oyster or in mussel. I K fast was inhibited by 4-AP, in agreement with data reported in many other species (Ö dblom et al 2000;Pennec et al 2004;Tseng et al 1987). Then TEA and 4-AP could be used to separate the fast and the delayed potassium currents in clam cardiomyocytes.…”

“…Such a calcium current, classically observed in vertebrate cardiac tissue (Hagiwara et al 1988) has been identified in other molluscs (Curtis et al 1999;Ö dblom et al 2000;Pennec et al 2004). L-type channel, in molluscs as in lower vertebrates, is thought to be the primary pathway for Ca 2?…”

“…Calcium-activated outward potassium current, activated by large depolarizing pulses and largely sensitive to calcium influx, was also observed in clam cardiomyocytes as in vertebrates (Siegelbaum and Tsien 1980;Kenyon and Sutko 1987;Giles and Imaizumi 1988) or invertebrates (Pennec et al 2004;Ö dblom et al 2000). This current has a role in the regulation of cell contractility and intracellular calcium regulation.…”

“…A sodium current was only found in few clam cells as evidenced in cardiac tissue of other molluscs (Pennec et al 2004;Curtis et al 1999;Ö dblom et al 2000). Varro and Papp (1992) showed that this current is responsible not only for the fast depolarization, but, in part also, for the maintenance of the plateau phase of the action potential.…”

Characterization of the voltage-activated currents in cultured atrial myocytes isolated from the heart of the common oysterCrassostrea gigas

“…More recently, two types of potassium channels and also two types of calcium channels were characterized in the heart of the same species. Three major potassium currents were identified in a preliminary report in the squid Loligo forbesii (Ödblom and Williamson, 1995), followed more recently by another species, Allotheutis subulata (Ödblom et al, 2000): a fast inactivating current that is inhibited by 4-amino pyridine (4-AP), a delayed outward rectified current that is inhibited by tetraethyl ammonium chloride (TEA), and a calciumactivated current that is inhibited by cobalt ions and apamin. Calcium and fast sodium channels were also found.…”