Mechanisms of Signal Transduction During a2-Adrenergic Receptor-Mediated Contraction of Vascular Smooth Muscle

Little is known about the signaling pathways involved in tr2-adrenergic receptor-mediated contraction of vascular smooth muscle. In the present study, we measured intracellular Ca21 ([Ca"2];), myosin light chain (MLC) phosphorylation, and myofilament Ca2' sensitivity during stimulation with the relatively selective cr2-agonist UK 14304. These effects were compared and contrasted with corresponding changes during depolarization by elevation of the [K'] in the bathing medium. These studies were performed using spiral strips of the rabbit saphenous vein, a tissue with a relatively high density of postsynaptic cv2-receptors. UK 14304 (10` M) caused parallel changes in [Ca2+1i, MLC phosphorylation, and force consisting of an initial phasic, followed by a sustained steady-state response. The steady-state increase in [Ca2J1i, MLC phosphorylation, and force caused by UK 14304 in the presence of 2.5 mM extracellular Ca were indistinguishable from those during 51 mM K' depolarization. However, when extracellular Ca was removed in the presence of UK 14304, [Ca211 and MLC phosphorylation fell to resting levels, but force remained significantly elevated above basal levels. UK 14304 caused no change in the steady-state [Ca211i-MLC phosphorylation relation. Thus, the [Ca 21]; sensitization of force was not caused by a sensitization ofMLC phosphorylation. These results indicate that in a 2.5-mM Ca2' bathing medium, the dominant mechanism by which a2-adrenergic receptor stimulation causes an increase in vascular tone is through a relatively large increase in [Ca21]1 and MLC phosphorylation. However, in Ca2'-free bathing medium, a second mechanism is unmasked which appears to involve an increased Ca21 sensitivity and is independent of myosin phosphorylation. (Circulation Research 1993;72:778-785)