Stimulation of Membrane Phospholipid Metabolism by Agents that Increase Acidification in Toad Urinary Bladder 1

Abstract

Previous reports have indicated that metabolic acidosis stimulates H⁺ excretion, and this excretion is accompanied by an increased turnover of phospholipids (PL) in toad urinary bladder. The purpose of this experiment was to determine if other known stimulators of H⁺ excretion [insulin, deoxycorticosterone acetate (DOCA), epinephrine, parathyroid hormone, and CO₂] might also stimulate PL turnover in the toad urinary bladder. Quarter bladders from normal toads were removed, weighed, and then incubated with [³²P]orthophosphate for 2 hr at 25°C. PL were extracted, separated, and detected using thin layer chromatography and autoradiography, and quantitated by liquid scintillation counting. Results were expressed in cpm (100 mg bladder)^-1 (hr)^-1. One quarter bladder received insulin (100 milliunits/ml), DOCA (10⁶ M), epinephrine (50 mM), parathyroid hormone (100 μg/ml), or 5% CO₂ during the incubation, whereas the paired quarter bladder received no treatment. Phosphatidylcholine (PC) and phosphatidylinositol turnover were increased by insulin (P < 0.025 and < 0.05, respectively). DOCA had no effect on PL turnover, but stimulated the percentage fraction of PC (P < 0.05) expressed as percentage fraction of total lipids. Five percent CO₂ in the bath resulted in an increased rate of turnover of the PL fractions phosphatidylinositol (P < 0.05), and the phosphatidic acid plus phosphatidyl-serine (P < 0.01). Epinephrine and parathyroid hormone were both without effect on PL metabolism. We conclude that insulin, DOCA, and CO₂ may stimulated H⁺ excretion in toad bladder in part by increasing turnover of membrane PL, PC, and phosphatidylinositol, and in the case of CO₂, phosphatidic acid plus phosphatidylserine as well, but not PC.