Voltage-gated calcium channels are involved in the regulation of calcium oscillations in vascular smooth muscle cells from isolated porcine retinal arterioles

Disturbances in the regulation of retinal arteriolar tone are involved in the pathophysiology of major vision threatening diseases. Therefore, improving the treatment of these diseases requires knowledge about how the contraction of retinal arterioles is controlled. The present study hypothesized that in addition to calcium release from sarcoplasmic reticulum, calcium entry from the extracellular space is also involved in the generation of calcium oscillations as a basis for initiation of contraction in retinal arterioles. Isolated porcine retinal arterioles were mounted in a confocal microvascular myograph and loaded with the calcium sensitive fluorophore Oregon Green allowing simultaneous measurements of oscillations in the intracellular calcium concentration in individual smooth muscle cells and changes in the vascular tone. Recordings were made after pre-contraction with the thromboxane analogue U46619 in a low and a high ambient calcium concentration, and after subsequent blocking of voltage-gated calcium channels using nifedipine or the sarcoplasmic reticulum dependent calcium recruitment using ryanodine, caffeine, thapsigargin and cyclopiazonic acid. U46619 increased the number of oscillations in a low but not in physiological extracellular calcium concentration. In contrast to mesenteric arterioles, blocking of sarcoplasmic reticulum Ca2+ channels with ryanodine was without effect on both retinal arteriolar tone and calcium oscillations. Conversely, blocking of the sarcoplasmic reticulum Ca2+ pump with cyclopiazonic acid or thapsigargin significantly reduced calcium oscillations, but not arteriolar tone. At a physiological extracellular calcium concentration, addition of nifedipine, an L-type calcium channel blocker, reduced vascular tone but increased the number of cells displaying calcium oscillations. Fast oscillations in the intracellular calcium concentration in retinal smooth muscle cells depend on calcium transport by both the SR and the sarcolemma. However, both sites of calcium recruitment seem to have unique properties in porcine retinal arterioles as compared to other organs.