Ca2+-sensitivity and cGMP-independent effects of NO in vascular smooth muscle

The effects of NO on Ca2+-sensitivity of vascular smooth muscle (VSM) myofilaments have been the focus of this study. Simultaneous measurements of [Ca2+]i and force were carried out in rat tail artery segments. NO, 10−7 M, evoked a transient decrease in [Ca2+]i accompanied by sustained relaxation (45.3 ± 6.3 vs. 69.45 ± 7.2%, P < 0.05, respectively) of VSM precontracted with K+ (70 mM), suggesting a decrease in Ca2+-sensitivity of VSM. This decrease in Ca2+-sensitivity was completely abolished by preincubation of VSM with ODQ (10−6 M) (63.9 ± 7.8% for [Ca2+]i vs. 20.5 ± 8.4% for relaxation, P < 0.05). Ca2+-presensitization of VSM myofilaments with PE (10−6 M) decreased the efficacy of NO to relax VSM (44.25 ± 6.9% vs. 69.45 ± 7.2%, P < 0.05), but increased its ability to lower [Ca2+]i (70.5 ± 6.8% vs. 45.3 ± 6.3%, P < 0.05). Application of DTT (10−3 M) together with ODQ (10−6 M) to subtract possible cGMP-independent effects revealed the total suppression of both the relaxant responses and [Ca2+]i of VSM under high-K+ preactivation of VSM. The data indicate that NO not only relaxes VSM and lowers [Ca2+]i in K+-preactivated VSM, but also decreases Ca2+-sensitivity of VSM myofilaments and these effects are strongly cGMP-dependent. In PE-induced contractions of VSM, NO relaxed VSM of rat tail artery and lowered [Ca2+]i, but failed to reverse Ca2+-presensitized myofilaments. We suggest that alternative cGMP-independent effects of NO are primarily manifested via activation of K+-channels and inhibition of Ca2+ current rather than to affect relaxation. An importance of reduced SH-groups within VSM myoplasm for both relaxation and [Ca2+]i disposal evoked by NO is evident whatever Ca2+-mobilization pathways are involved.