| Research Abstract |
Possible contribution of protein kinase C (PKC) to the cardiac pacemaker activity was investigated using sinoatrial and atrioventricular node cells isolated from rabbits. The membrane currents and action potentials were measured with the patch clamp method. In order to activate the PKC, angiotensin II, endothelin and a phorbol ester TPA were tised. Angiotensin II and endothelin significantly reduced the L-type Ca current I_<Ca> in sinoatrial node cells, and reduced the spontaneous firing rate. This inhibition of the I_<Ca> was inhibited by a protein kinase A inhibitor or dialysis of the cells with extrinsic cAMP. A pretreatment with pertussis toxin also abolished the effects of angiotensin II and endothelin. The angiotensin II-induced inhibition of I_<Ca> was dose-dependent, however at higher concentrations (【greater than or equal】30 nM), the initial inhibition was followed by a graduai increase in the I_<Ca>. TPA, a protein kinase activator, gradually increased the I_<Ca> by approxima
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tely 15%. This time course resembled the gradual increase in I_<Ca> observed with high concentrations of angiotensin II. The pretreatment with TPA did not reduce the effects of angiotensin II and endothelin. Acetylcholine also inhibited the I_<Ca> in these pacemaking cells, and angiotensin II did not show any additional effect to acetylcholine. According to these results, it is concluded that activation of protein kinase C enhances I_<Ca>, which can cause a positive chronotropic effect. However, the receptors which are coupled with the phospholipase C-protein kinase pathway are also linked with the pertussis toxin-sensitive G_i-adenylate cyclase pathway. Thus, application of these vasoactive peptides reduces the heart rate by lowering the intracellular cAMP concentration. This negative chronotropic effect is attenuated with time presumably through the desensitization of the receptor and PKC-induced potentiation of I_<Ca>. Long-term effects of PKC on the ion channels remain to be resolved. Less
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