Question:

Nitric oxide (NO) is a powerful vasodilator. The dilating effect of NO has been shown to be mediated by several mechanisms, in particular by an activation of vascular smooth muscle high conductance calcium-activated potassium channels (BK channels). The latter mechanism has been identified when changes in vessel tone due to an acute release of NO was studied at a persistent vasoconstrictor tone. In contrast, in vivo quite often changes in vessel tone are evoked by an acute action of a vasoconstrictor at a persistent influence of NO. Under these conditions, changes in vessel tone are often reduced, demonstrating the anticontractile effect of NO. Whether the anticontractile effect of NO also involves BK channels is unknown. Thus, the hypothesis was tested that BK channels facilitate the anticontractile effect of NO.

Methods:

Experiments were performed on rat tail arteries. Single BK channel activity and BK currents of intact cells were studied using the patch-clamp technique, vessel reactivity using isometric myography and intracellular calcium concentration changes using FURA-2 fluorimetry. Acute changes in vasoconstrictor tone were induced by the α₁-receptor agonist methoxamine.

Results:

Methoxamine (MX) constricted the arteries in a concentration-dependent manner. MX-induced contractions in the presence of the NO-donor SNP were weaker compared to contractions in the absence of SNP, demonstrating the anti-contractile effect of SNP. This anti-contractile effect of SNP was smaller in vessels with functional BK channels compared to vessels with non-functional BK channels (blocked by their specific inhibitor iberiotoxin (IBTX)). Functional BK channels in these arteries required calcium release from intracellular stores. The content of the intracellular calcium stores was considerably reduced by SNP. In whole-cell experiments with strongly buffered intracellular calcium, the outward current was increased by SNP. This effect was not observed in the presence of IBTX, hydroxocobalamin, a NO-scavenger, and Rp-8-Br-PET-cGMPS, a specific PKG-inhibitor. In inside-out patches, the activity of BK channels was increased by SNP, but also by addition of the experimental bath solution.

Conclusions:

Our study demonstrates that in rat tail arteries (i) the NO-donor SNP has no specific effect on single BK channels, (ii) SNP stimulates BK currents only in the presence of strongly buffered intracellular calcium and (iii) the anticontractile effect of SNP is limited by BK channels.