Calcium-independent release of endothelial nitric oxide in the arteriolar network: Onset during rapid juvenile growth

Objective: In young rats, skeletal muscle arterioles respond to acetylcholine, which elicits Ca(2+)-dependent endothelial nitric oxide ( NO) release, but not to shear stress, which does not require increased intracellular Ca(2+) for endothelial NO release. The aim of this study was to determine if, in general, signaling pathways for endothelial NO release that differ in their reliance on Ca(2+) may be developing at different times during normal arteriolar network growth in skeletal muscle. Methods: Arteriolar responses to intraluminal infusion of the Ca(2+) ionophore A23187, and the Ca(2+)-independent agonists vascular endothelial growth factor ( VEGF) and simvastatin, were studied before and during NO synthase (NOS) inhibition with N(G)-monomethyl-L-arginine (L-NMMA) in the exteriorized spinotrapezius muscle of weanling ( age 4 - 5 wks) and juvenile ( 7 - 8 wks) rats. Results: There were no age-dependent differences in arteriolar responses to A23187 applied over a concentration range that dilated arterioles from 9 +/- 2 to 74 +/- 8% of maximum, and L-NMMA attenuated these responses by the same amount in both age groups. In juveniles, arterioles dilated up to 39 +/- 5% of maximum in response to VEGF, and up to 83 +/- 6% of maximum in response to simvastatin, with L-NMMA greatly reducing the responses to both agonists. In contrast, arterioles in weanlings did not dilate in response to either agonist. Conclusions: These findings suggest that some Ca(2+)-independent signaling pathways for endothelial NO release may not initially be operational in the arteriolar network, but quickly become established during juvenile growth. This is consistent with the idea that microvascular control mechanisms are not fixed at birth, but rather undergo progressive changes in concert with microvascular network growth and changes in tissue metabolic requirements.