RELATIONSHIP BETWEEN CYTOSOLIC CA2+ LEVEL AND CONTRACTILE TENSION IN CANINE BASILAR ARTERY OF CHRONIC VASOSPASM

IN ORDER TO study the role of the Ca2+/calmodulin/myosin light chain k inase system in the development of chronic vasospasm caused by subarac hnoid hemorrhage, the cytosolic Ca2+ level ([Ca2+]i), measured with fu ra-2, a fluorescent Ca2+ indicator, and contractile tension were measu red simultaneously, and their quantitive correlation was examined in b asilar arterial tissue obtained from the canine ''two-hemorrhage'' mod el. Sixteen adult mongrel dogs were divided into two groups, control ( n = 8) and vasospasm (n = 8), and were killed 7 days after the first e xperimental subarachnoid hemorrhage. In basilar arterial tissue loaded with fura-2, 1) [Ca2+]i in the resting condition was not significantl y different between the two groups; 2) the increment in [Ca2+]i induce d by 40 mmol/L K+ stimulation was significantly smaller in vasospastic tissue (P < 0.01); and 3) 40 mmol/L K+-induced tension development pe r cross-sectional area for a fixed increment in [Ca2+]i was significan tly greater in vasospastic tissue (P< 0.01). In tissue not loaded with fura-2, active myogenic tone, which was tentatively represented by th e ratio of tonus relaxed with 10(-4) mol/L papaverine to initial resti ng tone, was significantly greater in vasospastic tissue (P< 0.05). Th ese findings, coupled with recent reports concerning the ratio of phos phorylated myosin light chain, indicate the following about vasospasti c arterial tissue in vitro: 1) in the resting condition, augmented myo genic tone, which is not accompanied by [Ca2+]i elevation, is probably not attributable to the Ca2+/calmodulin/myosin light chain kinase sys tem, and by contrast, 2) at a higher level of [Ca2+]i, a stronger cont raction for a fixed increment in [Ca2+]i is induced by Ca2+ sensitizat ion mechanisms of the calmodulin/myosin light chain kinase system.