Differentiation of alpha adrenoceptors mediating increase of oxygen consumption in rat submandibular salivary gland slices.

Clonidine, noradrenaline and adrenaline (in the presence of propranolol), but not phenylephrine and methoxamine, stimulated an increase in the oxygen consumption of these slices that was blocked by yohimbine but not by prazosin. The stimulation was inhibited by ouabain and required the presence of Ca2+ in the incubation medium. The calcium ionophore A 23187 stimulated oxygen consumption in the tissue slices and enhanced the respiratory effect of clonidine. Atropine and (D-Pro2, D-Trp7.9)-substance P failed to block the respiratory response to clonidine in concentrations that inhibited the respiratory effects of carbachol and substance P, respectively. Release of acetylcholine from the unstimulated gland slices was reduced by clonidine or Ca2+ omission. Yohimbine prevented the clonidine effect and stimulated acetylcholine resting release. Nifedipine did not affect either the release of acetylcholine or the clonidine-induced reduction of acetylcholine release but blocked the oxygen uptake due to clonidine or to release acetylcholine.