Cholinergic effect on rat proximal convoluted tubule

The effect of cholinergic agents on proximal tubular absorption of bicarbonate and fluid were examined to investigate the possible role of the cholinergic receptor in the regulation of renal function. Proximal convoluted tubule (PCT) and peritubular capillaries were perfused with bicarbonate-Ringer's solution containing radioactive inulin. Bicarbonate (total CO₂) was determined by microcalorimetry. The rates of bicarbonate absorption ( ) and fluid absorption (J_v) were 143.3±7.2 pEq/min · mm and 2.52±0.23 nl/min · mm, respectively. Addition of carbachol (10⁻⁸ M) to the capillary perfusate reduced by 17% and J_v by 32%. A higher dose of carbachol (10⁻⁶ M) did not further inhibit or J_v. Simultaneous perfusion of atropine (10⁻⁵ M) together with carbachol (10⁻⁸ M) abolished the inhibitory effect of carbachol on PCT transport. Atropine itself, however, had no effect on PCT transport. The inhibitory effect of carbachol was also diminished by lanthanum chloride (10⁻⁴ M). Carbachol (10⁻⁶ M) had no effects from the luminal side. W-7, a calmodulin antagonist, inhibited carbachol-induced effects. Ionophore A-23187 also inhibited J_v and . However, there was no additive effect when A-23187 and carbachol were combined in the capillary perfusate. These results suggest that there are functional cholinergic receptors on the basolateral side of the PCT that can regulate and J_v. Calcium influx may play a role in mediating the cholinergic effects on PCT transport.