Evidence that additional mechanisms to cyclic GMP mediate the decrease in intracellular calcium and relaxation of rabbit aortic smooth muscle to nitric oxide

Abstract

1. The role of cyclic GMP in the ability of nitric oxide (NO) to decrease intracellular free calcium concentration [Ca²⁺]_i and divalent cation influx was studied in rabbit aortic smooth muscle cells in primary culture. In cells stimulated with angiotensin II (AII, 10⁻⁷ M), NO (10⁻¹⁰–10⁻⁶ M) increased cyclic GMP levels measured by radioimmunoassay and decreased [Ca²⁺]_i and cation influx as indicated by fura-2 fluorimetry.

2. Zaprinast (10⁻⁴ M), increased NO-stimulated levels of cyclic GMP by 3–20 fold. Although the phosphodiesterase inhibitor lowered the level of [Ca²⁺]_i reached after administration of NO, the initial decreases in [Ca²⁺]_i initiated by NO were not significantly different in magnitude or duration from those that occurred in the absence of zaprinast.

3. The guanylyl cyclase inhibitor, H-(1,2,4) oxadiazolo(4,3-a) quinoxallin-1-one (ODQ, 10⁻⁵ M), blocked cyclic GMP accumulation and activation of protein kinase G, as measured by back phosphorylation of the inositol trisphosphate receptor. ODQ and Rp-8-Br-cyclic GMPS, a protein kinase G inhibitor, decreased the effects of NO, 10⁻¹⁰–10⁻⁸ M, but the decrease in [Ca²⁺]_i or cation influx caused by higher concentrations of NO (10⁻⁷–10⁻⁶ M) were unaffected. Relaxation of intact rabbit aorta rings to NO (10⁻⁷–10⁻⁵ M) also persisted in the presence of ODQ without a significant increase in cyclic GMP. Rp-8-Br-cyclic GMPS blocked the decreases in cation influx caused by a cell permeable cyclic GMP analog, but ODQ and/or the protein kinase G inhibitor had no significant effect on the decrease caused by NO.

4. Although inhibitors of cyclic GMP, protein kinase G and phosphodiesterase can be shown to affect the decrease in [Ca²⁺]_i and cation influx via protein kinase G, these studies indicate that when these mechanisms are blocked, cyclic GMP-independent mechanisms also contribute significantly to the decrease in [Ca²⁺]_i and smooth muscle relaxation to NO.

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