Stimulation of I(Ca) by basal PKA activity is facilitated by caveolin-3 in cardiac ventricular myocytes()

L-type Ca channels (LTCC), which play a key role in cardiac excitation–contraction coupling, are located predominantly at the transverse (t-) tubules in ventricular myocytes. Caveolae and the protein caveolin-3 (Cav-3) are also present at the t-tubules and have been implicated in localizing a number of signaling molecules, including protein kinase A (PKA) and β(2)-adrenoceptors. The present study investigated whether disruption of Cav-3 binding to its endogenous binding partners influenced LTCC activity. Ventricular myocytes were isolated from male Wistar rats and LTCC current (I(Ca)) recorded using the whole-cell patch-clamp technique. Incubation of myocytes with a membrane-permeable peptide representing the scaffolding domain of Cav-3 (C3SD) reduced basal I(Ca) amplitude in intact, but not detubulated, myocytes, and attenuated the stimulatory effects of the β(2)-adrenergic agonist zinterol on I(Ca). The PKA inhibitor H-89 also reduced basal I(Ca); however, the inhibitory effects of C3SD and H-89 on basal I(Ca) amplitude were not summative. Under control conditions, myocytes stained with antibody against phosphorylated LTCC (pLTCC) displayed a striated pattern, presumably reflecting localization at the t-tubules. Both C3SD and H-89 reduced pLTCC staining at the z-lines but did not affect staining of total LTCC or Cav-3. These data are consistent with the idea that the effects of C3SD and H-89 share a common pathway, which involves PKA and is maximally inhibited by H-89, and suggest that Cav-3 plays an important role in mediating stimulation of I(Ca) at the t-tubules via PKA-induced phosphorylation under basal conditions, and in response to β(2)-adrenoceptor stimulation.