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G protein-mediated FMRFamidergic modulation of calcium influx in dissociated heart muscle cells from squid, Loligo forbesii

Abdesslam Chrachri, Maria Ödblom and Roddy Williamson

1. The actions of the neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH₂) on the L-type (I_Ca,L) and T-type (I_Ca,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 µM). 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 (GTPS, 100 µM), a non-hydrolysable GTP analogue, mimicked the FMRFamide inhibition of the Ca²⁺ current and occluded any further FMRFamide-induced inhibition. Internal dialysis of these cells with guanosine 5'-O-(2-thiodiphosphate) (GDPS, 100 µM) reduced the FMRFamide-induced inhibition of the peak Ca²⁺ current. The inhibitory effects of FMRFamide were abolished by pre-incubation of the cells with pertussis toxin (200 ng ml^-1).

4. The activation kinetics of I_Ca,L were not affected by FMRFamide application, nor by internal perfusion with GTPS, and the FMRFamide-induced reduction in I_Ca,L was not relieved by large depolarising prepulses. These data indicate that FMRFamide can modulate I_Ca,L, but not I_Ca,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 I_Ca,L seen in the type II heart muscle cells was not mediated by a PTX-sensitive G protein pathway.