In-vivo phosphorylation of the cardiac L-type calcium channel beta-subunit in response to catecholamines

In canine myocardium, the -subunit of the L-type Ca²⁺ channel is phosphorylated by cAMP dependent protein kinase in vitro as well as in vivo (Haase et al. FEBS Lett 335: 217–222, 1993). We have assessed the identity of the -subunit as well as its in vivo phosphorylation in representative experimental groups of catecholamine-challenged canine hearts. Adrenergic stimulation by high doses of both noradrenaline and isoprenaline induced rapid (within 20 sec) and nearly complete phosphorylation of the Ca²⁺ channel -subunit. Phosphorylation in vivo was about 4-fold higher as compared to untreated controls. When related to catecholamine-depleted (reserpine-treated) hearts noradrenaline and isoprenaline increased the in vivo phosphorylation of the -subunit even 8-fold. This phosphorylation correlated positively with tissue levels of cAMP, endogenous particulated cAMP-dependent protein kinase (PKA) and the rate of contractile force development dP/dt_max. The results imply the involvement of a PKA-mediated phosphorylation of the Ca²⁺ channel -subunit in the adrenergic stimulation of intact canine myocardium.