Transient increases in diameter and [Ca2+]i are not obligatory for myogenic constriction

Studies were performed to determine the significance of temporal variation in vascular smooth muscle Ca2+ signaling during acute arteriolar myogenic constriction and, in particular, the importance of the stretch-induced intracellular Ca2+ concentration ([Ca2+](i)) transient in attaining a steady-state mechanical response. Rat cremaster arterioles (diameter similar to 100 mu m) were dissected from surrounding tissues, and vessel segments were pressurized in the absence of intraluminal flow. For [Ca2+](i) measurements, vessels were loaded with fura 2 and fluorescence emitted by excitation at 340 and 380 am was measured using video-based image analysis. Ca2+ and diameter responses were examined after increases in intravascular pressure were applied as an acute step increase or a ramp function. Additional studies examined the effect of longitudinal vessel stretch on [Ca2+](i) and arteriolar diameter. Step increase in intraluminal pressure (from 50 to 120 mmHg) caused biphasic change in [Ca2+](i) and diameter. [Ca2+](i) transiently increased to 114.0 +/- 2.0% of basal levels and subsequently declined to 106.7 +/- 4.4% at steady state. Diameter initially distended to 125.4 +/- 2.1% of basal levels before constricting to 71.1 +/- 1.2%. In contrast, when the same pressure increase was applied as a ramp function (over 5 min) transient vessel distension and transient increase in [Ca2+](i) were prevented, yet at steady state vessels constricted to 71.3 +/- 2.5%. Longitudinal stretch resulted in a large [Ca2+](i) transient (158 +/- 19% of basal) that returned to baseline despite maintenance of the stretch stimulus. The data demonstrate that the initial vessel distension (reflecting myocyte stretch) and associated global [Ca2+](i) transient are not obligatory for myogenic contraction. Thus, although arteriolar smooth muscle cells are responsive to acute stretch, the resulting changes in myogenic tone may be more closely related to other mechanical variables such as wall tension.