Specific binding of prostaglandins E2 and F2alpha by membrane preparations from rat skin

Specific binding of [3H]prostaglandin E2 ([3H]PGE2) and [3H]prostaglandin F2 alpha ([3H]PGF2 alpha) to plasma membrane and smooth endoplasmic reticulum fractions prepared from rat skin was demonstrated by the Millipore filter assay system. Specific binding was greater in the smooth endoplasmic reticulum fraction. Maximum binding was attained in the presence of Ca++ (0.5 x 10(-3) M), pH 7.2, and at a temperature of 37 C. Unlabeled PGE2 at a concentration of approximately 700 x 10(-9) M inhibited binding to the smooth endoplasmic reticulum fraction by approximately 90%. Other unlabeled prostaglandins, PGE1, PGF2 alpha, and a PGE2 analog, 7-0-13-prostynoic acid, at the same concentration inhibited binding by 40%, 25%, and 30%, respectively. A variety of unlabeled fatty acids (palmitic, oleic, linoleic, eicosatrienoic, and eicosatetraenoic acids), at the higher concentration of approximately 1700 x 10(-9) M, inhibited binding less than 30%, suggesting the presence of specific receptors for PGE2 in the smooth endoplasmic reticulum. Similar results were obtained for the competitive binding studies with [3H]PGF2 alpha. Proteolytic digestion of the membrane fraction by trypsin and pronase, or boiling for 15 min caused marked inhibition of binding, suggesting that the receptors for PGE2 and PGF2 alpha have a protein component. The Scatchard plot analysis of the equilibrium-binding data of [3H]PGE2 and [3H]PGF2 alpha to normal smooth endoplasmic reticulum fractions revealed an apparent dissociation constant (Kd) of 1.1 x 10(-9) M with a binding site concentration of 75 x 10(-12) M for [3H]PGE2, and a Kd of 1,0 x 10(-9) M with a binding site concentration of 35 x 10(-12) M for [3H]PGF2 alpha. These data indicate a greater concentration of binding sites for PGE2 than PGF2 alpha in the normal skin smooth endoplasmic reticulum. On the other hand, analysis of smooth endoplasmic reticulum from skin of essential fatty acid-deficient rats revealed a Kd of 1.2 x 10(-9) M with a binding site concentration of 75 x 10(-12) M for [3H]PGE2, and a Kd of 2.2 x 10(-9) M with a binding site concentration of 175 x 10(-12) M for [3H]PGF2 alpha. The concentration of binding sites for PGF2 alpha in the smooth endoplasmic reticulum of the skin of these rats is increased 5-fold when compared to normal values, whereas that of PGE2 was not altered. These results suggest a possible alteration of PGF2 alpha specific binding to skin endoplasmic reticulum during the pathophysiological abnormalities that accompany essential fatty acid-deficiency syndrome.