Muscarinic Receptors and Hydrolysis of Inositol Phospholipids in Rat Cerebral Cortex and Parotid Gland

Exposure of rat brain or parotid gland slices to muscarinic receptor agonists stimulates a phospholipase C that degrades inositol phospholipids. When tissue slices were labelled in vitro with [3H]inositol, this response could be monitored by measuring the formation of [3H]inositol phosphates. Accumulation of inositol 1,4-biphosphate in stimulated brain slices suggests that polyphosphonositides are the primary targets for phospholipase C activity. Li+ (10 mM) in the medium completely blocked the hydrolysis of inositol 1-phosphate, partially inhibited inositol 1,4-bisphosphate hydrolysis, but had no effect on the hydrolysis of inositol 1,4,5-trisphosphate by endogenous phosphatases. Muscarinic receptor pharmacology was studied by measuring the accumulation of [3H]inositol 1-phosphate in the presence of 10 mM Li+. In experiments on brain slices, the response to carbachol was antagonised by atropine with an affinity constant of approximately 8.79 +/- 0.12. Dose-response curves to several muscarinic agonists were constructed using brain and parotid gland slices. The results are consistent with relatively direct coupling of low-affinity muscarinic receptors to inositol phospholipid breakdown in brain slices; full agonists were relatively more potent in the parotid gland compared with the brain. Explanations for these differences are suggested.