Vasoconstrictor Hormones in Intact Small Arteries

The initiation of receptor-mediated small artery contraction is dependent on inositol 1,4,5-trisphosphatestimulated release of stored calcium. The role of the other product of inositol lipid hydrolysis, 1,2-diacylglycerol, in maintaining contraction remains controversial. Therefore, we have determined the contractile response of rat subcutaneous small arteries (<300 ,m i.d.), when mounted as ring preparations in a myograph, to noradrenaline, angiotensin II, KCl-induced membrane depolarization, and a cell-permeable diglyceride, dioctanoylglycerol. In parallel experiments, the conversion of this diglyceride to dioctanoylphosphatidate was studied in 32P-labeled vessels. Dioctanoylglycerol produced a slow-onset sustained contraction that was dependent on extracellular calcium. This was accompanied by the generation of the lipid dioctanoylphosphatidate. Noradrenaline and KCI induced rapid-onset sustained contractions and increased the production of dioctanoylphosphatidate (75% and 91%, respectively). In addition, dioctanoylglycerol levels were reduced (41%) after noradrenaline stimulation, suggesting activation of diacylglycerol kinase. In contrast, the contractile response to angiotensin II was transient, and this agonist did not significantly affect the conversion of dioctanoylglycerol to phosphatidate. Noradrenaline markedly increased (fourfold) the formation of endogenous phosphatidate, whereas endogenous 1,2-diacylglycerol was increased (47%) with angiotensin II. These results demonstrate that phosphatidate formation is regulated by vasoconstrictor hormones during receptor-mediated contraction, independent of diglyceride mass. Modulation of the levels of lipid second messengers downstream from phospholipid hydrolysis may represent a mechanism by which agonists that act through the same signaling system produce different contractile responses. (Circulation Research 1993;72:1163-1171)