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Local and cellular Ca²⁺ transients in smooth muscle of pressurized rat resistance arteries during myogenic and agonist stimulation

Victor A. Miriel, Joseph R. H. Mauban, Mordecai P. Blaustein and W. Gil Wier

Confocal laser scanning microscopy was used to visualize Ca²⁺ transients in the vascular smooth muscle cells (VSMC) of intact, pressurized rat mesenteric resistance arteries loaded with fluorescent calcium indicators. Vasoconstriction was assessed by measuring inner arterial diameter. All arteries were studied at 70 mmHg intralumenal pressure and 37 °C.

In the control condition of myogenic tone the arteries were constricted to 62 % (n = 10) of their passive diameter (p.d.). The [Ca²⁺]_i in most VSMC of these arteries was constant over time. In a small percentage (< 10 %) of cells in each artery, [Ca²⁺]_i oscillated regularly. Local calcium transients (Ca²⁺ sparks) were observed in five arteries studied with confocal linescan imaging.

Activation of -adrenoceptors by phenylephrine (PE, 1·0 µM) induced further vasoconstriction of pressurized arteries (to 27 % of p.d.). In this condition, [Ca²⁺]_i oscillations were prominent in a large percentage (83 %) of the VSMC. The Ca²⁺ oscillations ranged in frequency from 4 to 22 min^-1, and were usually asynchronous between cells.

High [KCl]_o (65 mM) induced nearly comparable vasoconstriction to PE (37 % of p.d.) but [Ca²⁺]_i oscillated in only about 13 % of cells in each artery.

Block of L-type Ca²⁺ channels (with nifedipine) in arteries activated by PE caused nearly full vasodilatation, but did not abolish the Ca²⁺ oscillations. Subsequent block of the sarcoplasmic reticulum Ca²⁺ pump (with cyclopiazonic acid) abolished Ca²⁺ oscillations in all cells.

We conclude that Ca²⁺ entering VSMC via L-type Ca²⁺ channels has an obligatory role in force development, both in myogenic tone and during ₁-adrenoceptor activation. The oscillatory pattern of [Ca²⁺]_i that persists in the absence of Ca²⁺ entry via L-type Ca²⁺ channels is ineffective in activating contraction.

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