4. Conclusion

endothelial mechanisms through myoendothelial feedback. The current model of myoendothelial feedback involves flux of InsP3 from smooth muscle to endothelial cells to elicit localized increases in Ca2+, activation of IKCa channels and possibly NOS, to limit smooth muscle contractility [11, 91, 116]. This model is supported by ultrastructural and histochemical studies showing that in rat mesenteric and basilar, and hamster retractor feed arteries, MEGJ connexins and IKCa channels are in close spatial association with ER and InsP3 receptors within endothelial projections that extend through the internal elastic lamina to make contact with smooth muscle cells [11, 55, 91, 94]. In hamster retractor feed arteries, myoendothelial feedback is fully accounted for by EDH. The α1-adrenoceptor agonist phenylephrine induced localized, InsP3-mediated Ca2+ signaling events within endothelial projections and block of endothelial IKCa channels enhanced smooth muscle depolarization and vasoconstriction [11]. In rat basilar arteries in which NO makes a major contribution to myoendothelial feedback, smooth muscle depolarization to 5-HT was accompanied by IKCa channel-mediated endothelial hyperpolarization. Inhibition of IKCa channels, gap junctional communication, TRPC3 or NOS potentiated smooth muscle depolarization to 5-HT in a non-additive manner indicating that rather being distinct pathways, NO and endothelial IKCa channel activity are part of an integrated mechanism for the regulation of agonist-induced vasoconstriction [91]. In the latter study, Ca2+ signaling was not investigated and the link between IKCa channel activity and NO production was not defined. However, NOS has now been localized close to MEGJs [117] and in co-cultures stimulation of smooth muscle cells with phenylephrine leads to MEGJ specific NOS phosphorylation within endothelial cells to increase NO [118]. Also, in mouse mesenteric vessels, phenylephrine stimulated endothelial TRPV4 sparklets in an InsP3-dependent manner, to engage SKCa and IKCa channels as well as, to a lesser extent, NOS [17]. Thus, given the ability of IKCa channels to modulate endothelial Ca2+ dynamics [12, 113, 114], it may be proposed that activation of IKCa channels at MEGJs following stimulation of smooth muscle cells by GPCR agonists, may amplify dynamic Ca2+ signals to enhance NO production.

The majority of studies described in this chapter have been conducted on isolated resistance arteries which in in vivo would be part of branching network of resistance vessels supplied by feed arteries in which effective control of blood flow requires coordinated behaviour amongst arterial segments [119]. As described above, diffusible mediators such as NO act locally to increase arterial diameter. In contrast, KCa channel-mediated hyperpolarization leads to both local dilation and conduction of the response through the endothelium for distances of several millimeters. This conduction allows for coordination of changes in arterial diameter in multiple vessel segments and so optimizes blood flow [4, 119, 120]. That is not to say that diffusible mediators do not play a role in global blood flow regulation within vascular beds. A recent study of the vascular bed of the mouse gluteus maximus muscle revealed that NO and EDH provide complementary endothelial pathways for ascending vasodilatation to optimize oxygen delivery to the muscle. EDH of downstream arterioles conducts along the endothelium into proximal feed arteries to cause dilation, and NO is released in response to luminal shear

3. Local versus conducted responses

52 Calcium and Signal Transduction

stress which increases secondary to downstream dilatation [120].

It has become apparent over the past 15 years that endothelial Ca2+ signaling patterns underlie the engagement of effectors such as NOS and/or KCa channels. The physiological significance of these stimulus-specific signaling pathways is not just that they determine the mediator of vasodilation, but also the scope of the impact of each stimulus on blood flow. Stimuli which predominantly elicit release of diffusible mediators will elicit local vasodilation whereas those that initiate EDH have the potential to dilate multiple arterial segments and so affect tissue perfusion. Further work is required to determine if the patterns of Ca2+ signaling described here have widespread applicability, and how they are impacted by age, sex and cardiovascular risk factors. Investigation of how changes in the components of signaling microdomains contribute to the etiology of endothelial dysfunction in conditions such as diabetes and hypertension may lead to the identification of new therapeutic targets.
