Participation of intracellular Ca2+ stores in arteriolar conducted responses

We examined the role played by intracellular Ca2+ stores in conducted vasomotor responses induced by phenylephrine (PE) in isolated hamster cremasteric arterioles. When applied briefly ( approximately 1 s) to isolated, cannulated arterioles by using pressure-pulse ejection from a micropipette, PE produced a strong local vasoconstriction and a very small biphasic conducted response (a small constriction followed by a dilation) that propagated several hundred micrometers along the vessel length. The conducted vasomotion was associated with a monophasic elevation of the endothelial cell intracellular Ca2+ concentration ([Ca2+]i) at the site of stimulation, as measured with the Ca2+ indicator fura 2. The Ca2+ pump inhibitor thapsigargin was used to limit filling of Ca2+ stores in smooth muscle and endothelial cells. Thapsigargin reduced baseline diameter and elicited a strong dilator component at the local site while enhancing both the constrictor and dilator components of the PE-induced conducted response. The enhanced conducted constrictor component induced by thapsigargin was mimicked by extraluminal application of tetraethylammonium or charybdotoxin but not by iberiotoxin, apamin, glibenclamide, barium, or 4-aminopirydine. Thapsigargin increased the estimated basal endothelial cell [Ca2+]i by approximately 60 nM and converted the PE-induced change in [Ca2+]i from monotonic to biphasic with a late elevation of [Ca2+]i above baseline that coincided with the increased dilatory component of the conducted response. Luminal application of charybdotoxin plus apamin significantly reduced the dilatory component of the conducted response. These results indicate that intracellular Ca2+ stores play a dynamic role in regulating conducted vasomotor responses apparently through modulation of KCa channels in both cell types.