ON THE RELATION OF CALCIUM TO OVINE PAROTID SECRETION

¹ Howard Florey Institute of Experimental Physiology and Medicine, University of Melbourne, Parkville, Victoria 3052, Australia

The effects of Ca²⁺-active agents on ovine salivary flow rate and composition were measured in parotid glands under several conditions: no stimulation, submaximal stimulation of the secretomotor nerve, or stimulation by the cholinergic agents carbachol or bethanechol. Agents were infused into the arterial blood supply of parotid glands and those investigated were: calcium chloride, the calcium ionophores Bay K8644 and A23187, the calcium chelators EGTA and EDTA, the voltage-dependent calcium channel blocker verapamil and congeners, the calmodulin inhibitor trifluoperazine (TFP) and congeners. None of the agents affected the flow rate of saliva from unstimulated or pharmacologically stimulated glands. Increased plasma [Ca²⁺] and the ionophores did not affect salivary flow in nerve-stimulated glands. In nerve-stimulated glands, EGTA and TFP reduced salivary flow rate and verapamil increased it. The effect of EGTA was reversed by restoring plasma [Ca²⁺] to normal (1·0-1·2 mmol/l) or above, but the responses to TFP and verapamil were not reversed by increasing plasma [Ca²⁺]. In all three conditions of stimulation, infusions of EGTA, verapamil or TFP increased salivary [HPO₄^2-] and reduced [HCO₃^-] and pH. The ionophores had the opposite effects but increased plasma [Ca²⁺] had no effect. At the same time, EGTA, verapamil or TFP increased salivary [Na⁺ + K⁺], Bay K8644 had the opposite effect but increased plasma [Ca²⁺] had no effect. The osmolality of the saliva was not altered in any of these circumstances. Salivary [Ca²⁺] was increased by Ca²⁺ infusion and reduced by EGTA. Glandular blood flow increased with infusion of agents which increased salivary [HPO₄^2-], fell with infusion of ionophores, and was unchanged by increased plasma [Ca²⁺]. Thus, there appear to be three calcium-related activities in ovine parotid salivary gland in vivo: (1) salivary flow rate by action at the neuroeffector site, (2) salivary composition by alteration of the ratio of HPO₄^2-:HCO₃^-, and (3) rate of blood flow through the gland by altering vascular resistance.