The involvement of myosin regulatory light chain diphosphorylation in sustained vasoconstriction under pathophysiological conditions.

publisherSmooth muscle contraction is activated primarily by phosphorylation at Ser19 of the regulatory light chain subunits (LC20) of myosin II, catalysed by Ca(2+)/calmodulin-dependent myosin light chain kinase. Ca(2+)-independent contraction can be induced by inhibition of myosin light chain phosphatase, which correlates with diphosphorylation of LC20 at Ser19 and Thr18, catalysed by integrin-linked kinase (ILK) and zipper-interacting protein kinase (ZIPK). LC20 diphosphorylation at Ser19 and Thr18 has been detected in mammalian vascular smooth muscle tissues in response to specific contractile stimuli (e.g. endothelin-1 stimulation of rat renal afferent arterioles) and in pathophysiological situations associated with hypercontractility (e.g. cerebral vasospasm following subarachnoid hemorrhage). Comparison of the effects of LC 20 monophosphorylation at Ser19 and diphosphorylation at Ser19 and Thr18 on contraction and relaxation of Triton-skinned rat caudal arterial smooth muscle revealed that phosphorylation at Thr18 has no effect on steady-state force induced by Ser19 phosphorylation. On the other hand, the rates of dephosphorylation and relaxation are significantly slower following diphosphorylation at Thr18 and Ser19 compared to monophosphorylation at Ser19. We propose that this diphosphorylation mechanism underlies the prolonged contractile response of particular vascular smooth muscle tissues to specific stimuli, e.g. endothelin-1 stimulation of renal afferent arterioles, and the vasospastic behavior observed in pathological conditions such as cerebral vasospasm following subarachnoid hemorrhage and coronary arterial vasospasm. ILK and ZIPK may, therefore, be useful therapeutic targets for the treatment of such conditions

Mechanism Of Activation Of Isolated Rabbit Aorta By A Stable Analog Of Pgh2

The mechanism of activation of isolated rabbit aorta by the 9(alpha), 11(alpha) epoxymethano-derivative of PGH(,2) (U-44069) was examined using contractile tension measurements and (\u2745)Ca flux determinations. The results suggest that the PGH(,2) analog activates smooth muscle by causing the release of intracellularly bound calcium and stimulating calcium influx. The intracellular calcium pool which is sensitive to release by the analog was identified as a portion of the agonist-releasable pool which is depleted by norepinephrine and histamine, but not angiotensin II. The refilling of this intracellular calcium pool was slowed if the rate of calcium influx was reduced by D-600 and was accelerated if calcium influx was stimulated by KCl. The rate of tension development in response to KCl was slowed if the releasable pool was depleted and accelerated in the presence of an agonist capable of releasing this pool. This suggests that the internal calcium release process may play two roles in the activation of arterial smooth muscle: (1) the released Ca(\u27+2) plays a direct role in activation and (2) a decreased capacity of release sites to sequester influxing Ca(\u272+) allows more of the calcium influx to be utilized for activation. The U-44069-stimulated calcium influx exhibited two phases; on initial rapid phase which is transient and a second, sustained phase characterized by a slower rate of calcium entry. The initial phase of the U-44069-stimulated (\u2745)Ca influx was found to be dependent upon the (\u2745)Ca-labeling of an extracellularly bound calcium pool. When added to isolated aorta during the washout of extracellularly bound (\u2745)Ca, U-44069 caused a transient stimulation of (\u2745)Ca efflux which preceded the onset of contraction. It is suggested that the calcium influx mechanism activated by the PGH(,2) analog may involve the release of extracellular calcium bound near the cell surface concurrent with an increase in plasmalemmal calcium permeability

Activation mechanisms of human renal artery: Effects of KCl, norepinephrine and nitrendipine upon tension development and 45Ca influx

The activation of human vascular smooth muscle by KCl-induced depolarization or norepinephrine and the inhibition produced by nitrendipine were studied in the isolated human renal artery. The contractile response of arterial rings to 80 mM KCl was abolished when extracellular Ca 2+ was removed, and was inhibited by nitrendipine (IC 50 = 10 −8 M). In contrast, a residual, transient contractile response to norepinephrine remained when extracellular Ca 2+ was removed and the norepinephrine-induced contractions obtained in the presence of extracellular Ca 2+ were not blocked by nitrendipine. KCl caused a stimulation of 45Ca influx which was completely prevented by 10 −6 M nitrendipine. Norepinephrine also caused a stimulation of 45Ca influx; however, the norepinephrine-induced 45Ca influx was not prevented by 10 −6 M nitrendipine. These findings are consistent with the concept that depolarization-induced induced activation of the human renal artery is primarily dependent upon a stimulation of Ca 2+ influx; whereas activation by norepinephrine involves the release of intracellular Ca 2+ in addition to the activation of a separate, receptor-operated Ca 2+ influx pathway

Modification of the Renal Hemodynamic Response to Vasoconstrictors by Calcium Antagonists

Theoretical considerations suggest that the renal response to calcium antagonists may vary depending on the factors influencing basal vascular tone. Studies were conducted using the isolated perfused rat kidney to determine the response to calcium antagonists under conditions in which the determinants of renal vascular tone were accurately defined. With this model, calcium antagonists elicit vasodilation only in the presence of a vasoconstrictor. In this setting, however, the degree of vasodilation elicited depends on the nature of the vasoconstrictor employed. Thus, the reduction in renal perfusate flow (RPF) elicited by KCl-induced depolarization was completely reversed by the calcium antagonist, nitrendipine. In contrast, identical levels of vasoconstriction elicited by norepinephrine or angiotensin II were only partially reversed, suggesting that these agonists activate the renal vasculature by mechanisms that are more complex than membrane depolarization. Examination of the response of glomerular filtration rate (GFR) revealed that in the presence of norepinephrine and angiotensin II, nitrendipine exerted a preferential augmentation of GFR. Thus, concentrations that produced only modest effects on RPF increased GFR to levels equal to or exceeding control values. This selective augmentation of GFR did not occur during the renal vasoconstriction elicited by KCl. It is proposed that renal microvessels exhibit regional heterogeneity in regard to activation mechanisms and sensitivity to calcium antagonists. Calcium antagonists may selectively attenuate agonist-induced vasoconstriction of preglomerular vessels

Renal Hemodynamic Effects of Calcium Antagonists

Recently, attention has focused on the effects of calcium antagonists on renal function. When administered in vitro to the isolated perfused kidney, calcium antagonists exhibit consistent actions permitting characterization of their renal effects. Calcium antagonists do not affect the vasodilated isolated perfused kidney, but they do dramatically alter the response of the kidney to vasoconstrictor agents. In the presence of norepinephrine, calcium antagonists markedly augment glomerular filtration rate but produce only a modest improvement in renal perfusion. This preferential augmentation of glomerular filtration rate may be attributable to a selective vasodilation of preglomerular vessels. Although the clinical implications of such observations are not yet clear, preliminary studies in experimental animal models indicate that calcium antagonists may exert salutary effects on renal function in clinical settings that are characterized by impaired renal hemodynamics. The possible benefits of calcium antagonists in ameliorating the development of renal dysfunction in patients in whom there is increased risk for the development of acute renal insufficiency remain to be evaluated