Effects of ultraviolet radiation and angiotensin on calcium binding in rabbit aortic microsomes

The microsomal subcellular fraction was isolated from rabbit thoracic aorta by differential centrifugation between 3,000 g and 102,000 g. In one group of experiments, microsomes were incubated in the presence of angiotensin and the calcium binding was determined by liquid scintillation detection. In the other group, the incubation period with angiotensin included exposure to ultraviolet light for part of the time. Microsomes incubated with 10(-6) M angiotensin for 30 min demonstrated a 38% decrease in calcium binding below control level. When incubation with this same concentration of angiotensin was followed by a 20 min exposure to ultraviolet light (3660 A), the calcium binding increased 10% above control level. Irradiation in the absence of angiotensin for 20 min produced a 20% increase in calcium binding above control. Microsomes depleted of their endogenous catecholamine stores and not treated with any stimulating drug exhibited a 32% increase in calcium binding after irradiation with ultraviolet light. The effect of ultraviolet light on calcium binding was found to be positively correlated with the duration of irradiation. The enhanced microsomal binding of calcium due to ultraviolet light corresponds to the decrease in tension which this radiation produces in actively contracted aortic strips.