MASTOPARAN AND RAB3AL PEPTIDE POTENTIATION OF CALCIUM-INDEPENDENT SECRETORY ACTIVITY IN RAT MELANOTROPHS IS INHIBITED BY GDP-BETA-S

The whole-cell patch-clamp membrane capacitance measurement was used t o monitor secretory activity in rat melanotrophs, while rab3AL, putati ve effector domain peptides of Rab3 small GTPases (20-30 kDa), were in troduced into cytosol, In melanotrophs dialyzed with calcium free solu tions membrane capacitance tends to decrease slightly. This decrease i s further potentiated with GDP beta S (500 mu M). We found that rab3AL (100 mu M) stimulated secretory activity in the absence of calcium, T he rab3AL response was qualitatively comparable to the response to mas toparan (1 mu M), an activator of certain heterotrimeric GTP-binding p roteins, Interestingly, inclusion of GDP beta S (500 mu M) resulted in a blockade of both rab3AL and mastoparan induced responses, We conclu de that rab3AL and mastoparan induce calcium-independent stimulation o f secretory activity in rat melanotrophs by activation of a downstream heterotrimeric GTP-binding protein. (C) 1997 Federation of European B iochemical Societies.