Yoshihito Sakata

Phospholipid metabolism and prostacyclin synthesis in hypoxic myocytes

We observed that in hypoxic myocardial cells prostacyclin and arachidonic acid release increased and that during hypoxia phospholipid degradation also occurred. In order to clarify the mechanism of phospholipid degradation, we determined the activity of phospholipases A2 and C. We found that phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were markedly decreased and that lysophosphatidylcholine and lysophosphatidylethanolamine were increased. In contrast, there was only slight phosphatidylinositol degradation and no lysophosphatidylinositol elevation was observed. These results show that phospholipase A2 was activated in hypoxic myocytes and had substrate specificity towards PC and PE. To study phospholipase C activity, membrane phospholipids were labeled with [3H]choline, [3H]inositol or [3H]ethanolamine. The release of inositol was observed, but neither choline nor ethanolamine was released. In hypoxia, myocardial-cell phospholipase C has high substrate specificity towards phosphatidylinositol. The activation of phospholipases is closely related to the intracellular Ca2+ concentration; it is though that inositol polyphosphatides may regulate intracellular Ca2+. We determined how Ca2+ influx occurs in hypoxia. beta-Adrenergic blockade and Ca2+ antagonists markedly suppressed Ca2+ influx, phospholipase A2 activity, phospholipase C activity and cell death. However, the alpha 1-adrenergic blockade was less effective in suppressing these phenomena. These results suggest that in hypoxic myocardial cells Ca2+ influx mediated by beta-adrenergic stimulation activates phospholipases A2 and C, and that phospholipid degradation and prostacyclin release then occur.

Phospholipid metabolism in cardiomyopathic hamster heart cells.

We demonstrated that the activities of phosphatidylinositide-specific phospholipase C, inositol 1,4,5-trisphosphate (IP3) kinase, and IP3 phosphatase were enhanced in cardiomyopathic hamster hearts (BIO 14.6 and BIO 53.58) in comparison to control hamsters (F1b). Release of both arachidonic acid and prostacyclin was markedly enhanced by norepinephrine in the cardiomyopathic hamsters. Phospholipase C in heart has high substrate specificity to phosphatidylinositol. IP3 production was markedly enhanced in the cardiomyopathic hamsters. We also determined the intracellular calcium concentration, which was higher in BIO 53.58 hamsters than in BIO 14.6 hamsters at 5-20 weeks of age. There was no significant difference in the intracellular calcium level between F1b and BIO 14.6 hamsters at 5 weeks of age. These results suggest that phosphatidylinositol turnover stimulated by norepinephrine may produce high intracellular calcium levels in both BIO 14.6 and BIO 53.58 myocytes. In addition, in BIO 53.58 hamsters, some mechanism such as the sarcoplasmic reticulum, which controls the intracellular calcium level, may deteriorate in function. We concluded from these results that a prolonged high intracellular calcium level may lead to the death of BIO 53.58 myocytes.

Angiotensin II Potentiates Collagen Synthesis in the Hypertrophied Heart

In cardiac hypertrophy,collagen accumulation in the myocardial interstitium is associated with hypertrophy of cardiomyocytes [1]. Collagen remodeling causes impaired ventricular compliance,abnormal electrical conduction and impaired oxygenation of the cardio-myocytes [2].At present,factors that regulate cardiac collagen metabolism are not elucidated.Eghbali et al [3] found that cardiac fibroblasts are mainly responsible for the synthesis of fibrillar type I and III co11agen.Recent studies have demonstrated that angiotensin II (A II),in addition to its inotropic and chronotropic actions,may exert effects by acting as a growth factor in the car¬diovascular system and that there may be local autocrine or paracrine renin-angiotensin system in several tissues [4].There are no data concerning the effects of A II on cardiac nonmyocytes.In this study we examined the effects of A II on collagen synthesis by cardiac fibroblasts in normotensive and hypertensive rats.