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.

1. The actions of the neuropeptide FMRFamide (Phe-Met-Arg-Phe-NHµ) on the L-type (ICa,L) and T-type (ICa,T) calcium currents were investigated in muscle cells dissociated from the heart of squid, Loligo forbseii. 2. The heart muscle cells could be divided into type I and type II cells, on the basis of morphological differences in the dissociated myocytes. FMRFamide induced a substantial block of the L-type calcium current seen in type I cells; this inhibition was rapid, reversible and dose dependent (IC50 = 0·1 ìÒ). FMRFamide induced an increase in the amplitude of the L-type calcium current in the type II heart muscle cells, but had no effect on the T-type calcium current in either type of dissociated heart muscle cell, even at concentrations much higher than those found to affect the L-type calcium current. 3. Internal dialysis of isolated type I heart muscle cells with guanosine 5'-O-(3-thiotriphosphate (GTPãS, 100 ìÒ), a non-hydrolysable GTP analogue, mimicked the FMRFamide inhibition of the Ca¥ current and occluded any further FMRFamide-induced inhibition. Internal dialysis of these cells with guanosine 5'-O-(2-thiodiphosphate) (GDPâS, 100 ìÒ) reduced the FMRFamide-induced inhibition of the peak Ca¥ current. The inhibitory effects of FMRFamide were abolished by pre-incubation of the cells with pertussis toxin (200 ng ml¢). 4. The activation kinetics of ICa,L were not affected by FMRFamide application, nor by internal perfusion with GTPãS, and the FMRFamide-induced reduction in ICa,L was not relieved by large depolarising prepulses. These data indicate that FMRFamide can modulate ICa,L, but not ICa,T, in squid heart muscle cells, and that the underlying G protein pathway is dissimilar to that commonly associated with transmitter modulation of channel activity. 5. The FMRFamide-modulated increase in ICa,L seen in the type II heart muscle cells was not mediated by a PTX-sensitive G protein pathway. Keywords:

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