In order to investigate the effects of sympathomimetic drugs on the calcium currents, whole cell
patch clamp technique was used in ventricular cells of the guineapig. Single ventricular cells were
,´ -Isolated by Langendorff perfusion method with collagenase. Ttype and Ltype Ca--currents were
- recorded in different holding potentials. Ttype Cacurrent was activated from more negative mem-
t~´ brane potentials(aroud -50 mV) and was inactivated very rapidly, whereas Ltype Cacurrent was ,t
! activated from -30 mV and inactivated with two time constants(fast and slow components). Re-placement of calcium with barium increased the activation magnitude of L-type Ca-current and delayed inactivation time course, but did not affect Ttype Cacurrent significantly. Nifedipine blocked and Bay K 8644 increased Ltyp Cacurrent with voltagedependent manner. Ttype Cacurrent, however, was relatively incentive to dihydropyridine compounds. Isoproterenol´ which is known to have mainly beta-agonist increased L and Ttype Cacurrents and propranolol reversed the effect of isoproterenol. Phenylephrine(alphaagonist) and phentolamine(alpha-blocker) have no significant effects on the Ca-currents.
From the above results it is suggested that L-type and Ttype Cacurrents are present in guineaPig ventricular myocytes and are very sensitive to betaagonist or antagonist, but relatively insensitive to drugs acting on alphareceptors.
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