Alpha and beta adrenergic agonists stimulate water absorption in the rat proximal tubule

Simultaneous capillary and luminal microperfusion studies were performed in the rat proximal tubule to determine the effects of the beta agonist isoproterenol and the alpha agonist phenylephrine on water absorption. Capillary and luminal perfusion solutions were composed such that organic solutes were not present, no bicarbonate was present in the lumen, and no chloride gradient was imposed. Under such conditions, water absorption (Jv) averaged 0.36 +/- 0.11 nl . min-1 . mm-1. the addition of isoproterenol to the capillary solution in concentrations of 10(-6) and 10(-4) M resulted in significantly higher Jv's of 0.68 +/- 0.10 and 0.71 +/- 0.11 nl . min-1 . mm-1, respectively. The enhancing effect of isoproterenol was inhibited by the beta blocker propranolol (10(-4) M), but not by the alpha blocker phentolamine (10(-7) M). The addition of phenylephrine (10(-6) M) to the capillary perfusion solution also resulted in a significantly higher Jv of 0.84 +/- 0.14 nl . min-1, an effect inhibited by phentolamine (10(-7) M), but not by propranolol (10(-4) M). Neither phentolamine nor propranolol alone in the concentrations indicated had an effect on water absorption. These experiments indicate that both alpha and beta agonists stimulate water absorption in the superficial proximal tubule of the rat. This effect appears to be relatively specific for each class of agonist, as demonstrated by the effects of the specific antagonists.