Evidence that additional mechanisms to cyclic GMP mediate the decrease in intracellular calcium and relaxation of rabbit aortic smooth muscle to nitric oxide
Rm. Weisbrod et al., Evidence that additional mechanisms to cyclic GMP mediate the decrease in intracellular calcium and relaxation of rabbit aortic smooth muscle to nitric oxide, BR J PHARM, 125(8), 1998, pp. 1695-1707
1 The role of cyclic GMP in the ability of nitric oxide (NO) to decrease in
tracellular free calcium concentration [Ca2+](i) and divalent cation influx
was studied in rabbit aortic smooth muscle cells in primary culture. In ce
lls stimulated with angiotensin II (AII, 10(-7) M), NO (10(-10)-10(-6) M) i
ncreased 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 3-2
0 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 GRIP accumulation and activation of pr
otein kinase G, as measured by back phosphorylation of the inositol trispho
sphate 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 ki
nase 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 pro
tein kinase G, these studies indicate that when these mechanisms are blocke
d, cyclic GMP-independent mechanisms also contribute significantly to the d
ecrease in [Ca2+](i) and smooth muscle relaxation to NO.