Relative contributions of Ca²⁺ influx and cellular Ca²⁺ release during drug induced activation of the rabbit aorta

Using the "La-method" for measuring45Ca entry into smooth muscle cells we were able to show that the slow phase of a norepinephrine induced contraction is dependent on Ca2+ influx. Similarly Ca2+ influx was stimulated during the slow phase of angiotensin II and histamine induced contractions. The large rapid initial phases of these three types of drug induced aortic contractions were dependent on a common intracellular Ca2+ store which was rapidly exhausted in the presence of extracellular La3+. Relaxation of the aortic strips was not dependent on loss of Ca2+ from the cells indicating the presence of intracellular Ca2+ binding sites capable of lowering the cytoplasmic (Ca2+) to below 10-7 M. These latter sites are distinct from the intracellular Ca store released by norepinephrine, angiotensin and histamine.