Histamine concentration and Ca2+ mobilization in arterial smooth muscle

Smooth muscle contraction is dependent on Ca2+ entry from the extracellular space or release from intracellular stores. The sensitivity of these Ca2+ sources to agonist concentration was evaluated by measuring myoplasmic [Ca2+] (as estimated by aequorin), myosin phosphorylation, and isometric stress in the swine carotid media. High histamine concentrations produced transient elevations in [Ca2+] and phosphorylation with rapid generation of near maximal stress. Lower histamine concentrations produced much smaller [Ca2+] and phosphorylation transients, and stress development was slower. Peak [Ca2+] was proportional to the rate of stress development. Steady-state [Ca2+], phosphorylation, and stress values (which are dependent on extracellular Ca2+) were more sensitive to histamine concentration than was the peak [Ca2+] response both in the presence and absence of extracellular CaCl2 (measures of intracellular Ca2+ release). This result suggests that the mechanism for Ca2+ influx from the extracellular space is more sensitive to histamine than intracellular Ca2+ release. These results are also consistent with the hypothesis that agonist-releasable sarcoplasmic reticular Ca2+ is the major contributor to initial phosphorylation transients that enhance the rate of stress development.