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We studied the effects of hydrocortisone, progesterone, estradiol, and testosterone on phospholipase A 2 activity in plasma and lysosomal membranes of rat dermis and epidermis. The dermal enzyme was more sensitive to estradiol, while epidermal to testosterone. The effects of progesterone and hydrocortisone were mediated by lipocortin-like protein, while estradiol and testosterone directly by modulated phospholipase A 2 activity Key Words: skin; dermis; epidermis; estradiol; hydrocortisone; progesterone; testosterone; phospholipase A 2 Phospholipase A 2 (PLA2) catalyzing the release of fatty acids (primarily arachidonic acid) from phospholipids is localized in different cell membranes [3,4] and participates in the intracellular transduction of hormonal signals. PLA 2 activity is modulated by bioactive substances and radiation. Hydrocortisone has been reported to inhibit this enzyme [3], the effects of other steroid hormones have practically never been studied.This study investigated the effects of steroid hormones on PLA 2 activity in plasma and lysosomal membranes (PM and LM) from rat skin. MATERIALS AND METHODSExperiments were carried out on mature and immature rats (males and females in the diestrus phase verified as described elsewhere [1]). Hydrocortisone. progesterone, estradiol, and testosterone were administered subcutaneously (0.5 ml) in a dose of 10 mg/kg. The animals were sacrificed under light ether anesthesia 5 and 30 min, 1, 24, and 48 h postinjection. Skin homogenates were prepared [5]. PLA 2 activity was measured by the content of water-soluble radioactive products of phospholipid hydrolysis [8]. 1-Acyl-2.1.Jncarachnoidyl-glycero-3-phosphocholine (2.33 nCi/pg Department of Molecular Pharmacology and Radiobiology, MedicoBiological Faculty, Russian State Medical University, Moscow phospholipid) was used as the substrate. Protein content was measured according to Lowry [7]. The data were processed statistically using Student's t test. RESULTSAt the first stage we determined PLA z activity in dermal and epidermal PM and LM from intact rat skin. Since skin properties depend on age [2] and the course of many skin diseases depends on patient's gender, the age and sex of experimental animals were taken into account.In young mature rats, PLA 2 activity in dermal and epidermal PM showed no sex or tissue specificity (Table 1). The tissues of older rats (6 months) were characterized by lower PLA 2 activities in both PM and LM compared to young mature ( 1.5-2 months) animals. In the dermis and epidermis of immature 2-3-week-old rats, PLA 2 activity was found only in PM, being significantly (by 16-22%) higher than in young mature animals. Thus, in immature and old animals skin lysosomes are tess functionally active and hence the skin is poorly protected from microbial and other injuries.In the second experimental series, PLA 2 activity was measured at different times (5, 30, 60 min; 24 and 48 h) after subcutaneous administration of hormones in a dose of 10 mg/kg.The effects of hydrocortisone and progesterone (si...