Functional Study of the [Ca²⁺]_i Signaling Pathway in Aortas of <i>L</i>-NAME-Hypertensive Rats

Abstract

A variety of mechanisms has been proposed to suggest that nitric oxide participates in the regulation of smooth muscle free [Ca²⁺]_c (the primary determinant of contractile tone), including inhibition of Ca²⁺ influx across the plasma membrane and inhibition of intracellular Ca²⁺ release. In view of such considerations, the aim of this study was to investigate the possible alterations in contractile responses induced by drugs that mobilize Ca²⁺ from different sources in aortae from N^G-nitro-L-arginine methyl ester (L-NAME) hypertensive rats (LHR). Treatment with L-NAME did not alter the contractile response induced by phenylephrine; however, indomethacin increased the contraction to phenylephrine only in LHR aortae (1.36 ± 0.08 g, n = 6, vs. 1.97 ± 0.09 g, n = 7). Both phenylephrine and caffeine evoked rapid and phasic contractions in intact or denuded aortic rings in Ca²⁺-free solution containing EGTA. Phenylephrine-elicited phasic contractions were lower in normotensive rats (NR; 0.41 ± 0.05 g, n = 9) than in LHR (0.57 ± 0.06 g, n = 6) and were increased by endothelium removal only in the NR group (0.64 ± 0.05 g, n = 6). Conversely, neither with treatment with L-NAME nor endothelium removal altered the phasic contractile responses induced by caffeine. The Ca²⁺ influx stimulated with phenylephrine was greater in NR (1.95 ± 0.08 g; pD₂ 6.06 ± 0.69; n = 8) than in the LHR denuded aorta (1.63 ± 0.11 g; pD₂ 3.52 ± 0.06; n = 6). Similarly, contractions stimulated with phorbol ester in denuded arteries were greater in NR (1.76 ± 0.08 g, n = 7) than in LHR (1.11 ± 0.11 g, n = 7). In the same manner, indomethacin failed to alter the contraction stimulated with phorbol ester in NR arteries (2.01 ± 0.21 g, n = 7), although it completely blocked the inhibitory effect of chronic treatment with L-NAME on this contractile response (1.94 ± 0.24 g; n = 9). Indomethacin did not change the contractile responses stimulated by increasing concentrations of extracellular Ca²⁺ in either NR aortas (1.44 ± 0.26 g; pD₂ 4.74 ± 0.79; n = 6) or LHR aorta (1.99 ± 0.19 g; pD₂ 4.10 ± 0.47; n = 8). However, in the presence of indomethacin, the Ca²⁺ influx was similar in NR and LHR aortae. Taken together, these results suggest that, in this model of hypertension, the increase in agonist-induced release of Ca²⁺ from intracellular stores may be partly compensated by inhibition of Ca²⁺ influx and that this effect is due to the increased production of the relaxant prostanoid in vascular smooth muscle cells.

Effect of magnesium on vascular reactivity in NOS inhibition-induced hypertension

“…Our results demonstrated that KCl-induced constriction responses did not alter, whereas Phe-induced responses were greater in the aorta of H animals compared that those of the control group in the presence of PSS containing a normal magnesium concentration. Although some studies have demonstrated unchanged responses [20,21], an increase in the adrenergic vascular contraction response in the NOS inhibition-induced hypertension model have been shown by many reports [22][23][24][25]. In the present study, the PSS that contained a high magnesium concentration did not cause any differences in the constriction responses of aortic rings obtained from both normotensive and hypertensive animals (C-Mg and H-Mg).…”