The effect of ouabain and of Ca2+ deprivation on isoproterenol- and DBcAMP-stimulated protein secretion from the superfused rat parotid gland

Abstract

Protein secretion from rat parotid slices was studied by measuring the rate of release of 3H-protein from tissue superfused with Krebs–Ringer bicarbonate medium. Isoproterenol (IPR) and dibutyryl 3′5′ cyclic adenosine monophosphoric acid (DBcAMP) stimulated the release of protein from parotid slices in the presence of extracellular calcium (10−3 M). Ouabain (10−3 M) potentiated the secretory response to IPR (10−5 M) and inhibited the response to DBcAMP (10−3 M) but itself did not stimulate protein release. In the absence of extracellular calcium, IPR caused a significantly greater protein secretion (p < 0.05) than in the presence of that ion. In contrast DBcAMP did not stimulate protein secretion in the absence of extracellular calcium. Isoproterenol increased cAMP concentration in the tissue whereas ouabain had no effect on the tissue cAMP concentration in the absence or presence of IPR. In contrast with adrenaline, isoproterenol does not alter the ATP content of the tissue.The distribution of [3H]DBcAMP in the tissue was not significantly different from that of [14C]sorbitol. Removal of extracellular calcium (with the addition of 10−3 M EGTA) or the addition of ouabain had no effect on the distribution of DBcAMP.These results suggest that the stimulation of protein release by DBcAMP is an extracellular (plasma membrane) effect and that this nucleotide does not mimic the effect of isoproterenol. It is also concluded that altering the intracellular Na+:K+ ratio by inhibition of an ouabain-sensitive Na+–K+-ATPase does not of itself cause protein release, but does, under certain conditions, augment the response to isoproterenol perhaps by increasing a Na+-dependent mobilization of Ca2+ from intracellular stores.