Accumulation of phosphatidic acid in microsomes from propranolol-treated retinas during short-term incubations

The pool size and synthesis of phosphatidic acid derived from [2-3H]glycerol were studied in bovine whole retinas and subcellular fractions. Microsomal preparations from retinas incubated with [2-3H]glycerol displayed the highest percentage labeling of phosphatidic acid at 5 min of incubation: labeling decreased rapidly thereafter. In drug-treated retinas, 0.5 mM propranolol increased the endogenous content of phosphatidic acid and stimulated [2-3H]glycerol labeling in whole retina and microsomal and postmicrosomal supernatant fractions. This effect was observed during short-term incubations and was reversible. In pulse-chase experiments, 60 min of reincubation greatly reduced the labeling effect, although propranolol still enhanced phosphatidic acid labeling. At the same time, endogenous phosphatidic acid accumulated, and reincubation without propranolol reversed the effect. During accumulation, the amount of palmitate increased and that of oleate decreased, whereas the relatively high level of docosahexaenoate in phosphatidic acid remained unchanged. It was concluded that this propranolol-induced effect is due to cationic amphiphilic drug activity in the endoplasmic reticulum that results in a partial inhibition of phosphatidic acid degradation and a stimulation of its de novo synthesis. Hence, net synthesis of phosphatidic acid can be assessed in the retina during short-term incubation with propranolol.