Enhancement of contraction and L-type Ca2+ current by murrayafoline-A via protein kinase C in rat ventricular myocytes

We previously reported that murrayafoline-A (1-methoxy-3-methyl-9H-carbazole, Mu-A) increases the contractility of ventricular myocytes, in part, via enhancing Ca2+ influx through L-type Ca2+ channels, and that it increases the Ca2+ transients by activation of protein kinase C (PKC). In the present study, we further examined the cellular mechanisms for the enhancement of contractility and L-type Ca2+ current (ICa,L) by Mu-A. Cell shortening and ICa,L were measured in rat ventricular myocytes using a video edge detection method and perforated patch-clamp technique, respectively. We found that the positive inotropic effect of Mu-A was not affected by pre-exposure to the β-adrenoceptor antagonist propranolol, the protein kinase A (PKA) inhibitors KT5720 or H-89, or the phospholipase C inhibitor U73122. Interestingly, the Mu-A-mediated increases in cell shortening and in the rate of contraction were completely suppressed by pre-treatment with the PKC inhibitor GF109203X. The stimulatory effect of Mu-A on ICa,L was not altered by inhibition of PKA (KT5720), G-protein coupled receptors (suramin), or α1-adrenoceptor (prazosin). However, pre-exposure to the PKC inhibitor, GF109203X or chelerythrine, abolished the Mu-A-induced increase in ICa,L. Pre-exposure to the Ca2+-calmodulin-dependent protein kinase II (CaMKII) inhibitor KN93 slightly reduced the stimulatory effects on contraction and ICa,L by Mu-A. Phosphorylation of PKC was enhanced by Mu-A in ventricular myocytes. These data suggest that Mu-A increases contraction and ICa,L via PKC in rat ventricular myocytes, and that the PKC-mediated responses in the presence of Mu-A may be partly mediated by CaMKII.