Modulation of basal intracellular calcium by inverse agonists and phorbol myristate acetate in rat-1 fibroblasts stably expressing alpha(1d)-adrenoceptors

In rat-1 fibroblasts stably expressing alpha(1d)-adrenoceptors BMY 7378, ph entolamine, chloroethylclonidine and 5-methyl urapidil decreased basal [Ca2 +](i). WB 4101 induced a very small effect on this parameter but when added before the other antagonists it blocked their effect. All these agents inh ibited the action of norepinephrine. Phorbol myristate acetate also blocked the effect of norepinephrine and decreased basal [Ca2+](i). Staurosporine inhibited these effects of the phorbol ester, Our results suggest that: (1) alpha(1d)-adrenoceptors exhibit spontaneous ligand-independent activity, ( 2) BMY 7378, phentolamine, chloroethylclonidine and 5-methyl urapidil act a s inverse agonists and (3) protein kinase C activation blocks spontaneous a nd agonist-stimulated alpha(1d)-adrenoceptor activity. (C) 1999 Federation of European Biochemical Societies.