Ca2+ sensitivity of isolated arterioles from the hamster cheek pouch

When isolated from the hamster cheek pouch, cannulated, and perfused, 60- to 90-microns arterioles spontaneously contracted to 67 +/- 4% of maximum diameter. Vessel sensitivity to variations in extracellular Ca2+ was then evaluated. Tone, regardless of its source, was highly dependent on the concentration of Ca2+ in the bathing solution. The magnitude of responses to changing Ca2+ depended upon which vessel surface (luminal or abluminal) the change was made. For K(+)-induced tone the Ca2+ concentration-response curve was right shifted 60-fold for luminal vs. abluminal changes. These results suggest that restricted diffusion of Ca2+ from lumen to smooth muscle dramatically reduces smooth muscle Ca2+ concentration and that under standard in vitro conditions the smooth muscle layer is effectively isolated from luminal contents. Both the cytosolic and stored Ca2+ in these microvessels were dependent on the Ca2+ concentration in the bathing solution. Abrupt removal of Ca2+ from bath produced a rapid maximal dilation with a mean time to half-maximal response (t1/2 max) of 14 +/- 4 s. Ca2+ replacement induced a return to the previous level of tone with a mean t1/2 max of 8 +/- 3 s. The magnitude of transient responses to caffeine (10 mM) was inversely related to the time of exposure to zero Ca2+ with a rapid decay in magnitude (t1/2 max = 2.7 +/- 0.8 min). These data suggest that the smooth muscle cells of arterioles have a particularly rapid transmembrane Ca2+ flux that is tightly controlled by an intracellular regulatory mechanism, which may explain the generally increased dependence of smaller vessels on extracellular Ca2+.