Simultaneous capillary and luminal microperfusion studies in the proximal convoluted tubule of the rat were performed to examine the transepithelial secretory flux of [14C]oxalate. Increases in the concentration of oxalate in the capillary solution from 0.096 to 4.3 mM resulted in progressively higher rates of oxalate secretion into the lumen. Further increases in the capillary concentration of oxalate indicated a tendency toward a plateau. The inclusion of para-chloromercuribenzoate, sodium cyanide, indanyloxyacetic acid, furosemide, or para-aminohippurate in the capillary solution significantly lowered the secretory flux of oxalate. the addition of probenecid in a concentration of 10(-4) M inhibited oxalate secretion when the oxalate concentration in the capillary solution ranged between 1.1 and 4.3 mM, but did not affect oxalate secretion at higher capillary concentrations of oxalate. These results indicate that oxalate secretion in the rat proximal tubule is an active carrier-mediated process. When considered in conjunction with prior studies, the present investigations suggest the possibility that more than one oxalate secretory system exists 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.
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