In order to investigate the effect of intracellular cyclic GMP on the calcium channel, whole cell patch clamp technique with internal perfusion method was used in the single ventricular myocytes of the rabbit. Cyclic GMP, cGMP analogues, cAMP,
isopernaline and forskolin were perfused into cells and their effects on the calcium current were analysed by applying depolarizing step pulese of 10mV in amplitude for 200 msec from holding potential of -40mV. Calcium currents usually activated
from
-30mV and then reached a peak at +10mV. Amplitude of the calcium current was standardized with membrane capacitance, 50 pF. Peak amplitude at +10mV in control was -0.5 nA/50pF. When 100μM cAMP was applied from the pipette, peak amplitude of
calcium
current increased to -0.32 nA and addition of 1 μM isoprenaline further increased its amplitude. In the presence of cGMP it alone also produced an increase of the calcium current to -.5.2 nA/50pF and addition of isoprenaline or forskolin
increased
its
magnitude to -(0.55-0.95) nA/50pF. Simultaneous application of cGMP and cAMP increased the calcium current to -0.67 nA/50pF. Among the cGMP analogues. 8-Br-cGMP was the most potent stimulant for the calcium current activation.
From the above results it could be concluded that cGMP increases the calcium current not through cAMP dependent protein kinase nor cAMP dependent phosphodiesterase pathway, but through independent phosphorylation pathway, possibly cGMP dependent
protein
kinase pathway.
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