Calcium and vascular smooth muscle tone.

Recent technologic advances have improved the monitoring of intracellular ionized calcium concentrations ([Ca2+]i) in living vascular smooth muscle cells. The changes in cytoplasmic ionized calcium concentrations ([Ca2+]) that occur during a contraction-relaxation cycle of vascular smooth muscle are reviewed, as is recent evidence indicating that vascular smooth muscle tone can be increased by increasing the sensitivity of the contractile apparatus to calcium without a necessary change in [Ca2+]i levels. Data obtained using intracellular calcium indicators suggest that agonists that produce similar force profiles often produce very different profiles in [Ca2+]i. The amount of force produced at a specific level of [Ca2+]i is a variable of intact vascular smooth muscle, i.e., some agonists produce sizable tonic contractions with little elevation of cytoplasmic [Ca2+], whereas other agonists require much larger increases in cytoplasmic [Ca2+] to produce comparable increases in tone. These data also indicate that very small changes in [Ca2+]i can cause large changes in vascular tone. Designing therapeutic agents that would decrease [Ca2+]i or decrease the sensitivity of the contractile apparatus to ionized calcium would directly decrease intrinsic myogenic tone. Although a causative role for abnormal [Ca2+]i in the etiology of hypertension is controversial, it is obvious that a decrease in these levels in the vascular smooth muscle cell could be therapeutically beneficial.