KCl-depolarization potentiates the Ca2+ sensitization by endothelin-1 in canine coronary artery

Protein kinase C theta activation induces insulin-mediated constriction of muscle resistance arteries

OBJECTIVE

Protein kinase C (PKC) theta activation is associated with insulin resistance and obesity, but the underlying mechanisms have not been fully elucidated. Impairment of insulin-mediated vasoreactivity in muscle contributes to insulin resistance, but it is unknown whether PKC theta is involved. In this study, we investigated whether PKC theta activation impairs insulin-mediated vasoreactivity and insulin signaling in muscle resistance arteries.

RESEARCH DESIGN AND METHODS

Vasoreactivity of isolated resistance arteries of mouse gracilis muscles to insulin (0.02-20 nmol/l) was studied in a pressure myograph with or without PKC theta activation by palmitic acid (PA) (100 micromol/l).

RESULTS

In the absence of PKC theta activation, insulin did not alter arterial diameter, which was caused by a balance of nitric oxide-dependent vasodilator and endothelin-dependent vasoconstrictor effects. Using three-dimensional microscopy and Western blotting of muscle resistance arteries, we found that PKC theta is abundantly expressed in endothelium of muscle resistance arteries of both mice and humans and is activated by pathophysiological levels of PA, as indicated by phosphorylation at Thr(538) in mouse resistance arteries. In the presence of PA, insulin induced vasoconstriction (21 +/- 6% at 2 nmol/l insulin), which was abolished by pharmacological or genetic inactivation of PKC theta. Analysis of intracellular signaling in muscle resistance arteries showed that PKC theta activation reduced insulin-mediated Akt phosphorylation (Ser(473)) and increased extracellular signal-related kinase (ERK) 1/2 phosphorylation. Inhibition of PKC theta restored insulin-mediated vasoreactivity and insulin-mediated activation of Akt and ERK1/2 in the presence of PA.

CONCLUSIONS

PKC theta activation induces insulin-mediated vasoconstriction by inhibition of Akt and stimulation of ERK1/2 in muscle resistance arteries. This provides a new mechanism linking PKC theta activation to insulin resistance.

A new view of K+ -induced contraction in rat aorta: the role of Ca2+ binding

Strong, K+ -induced contractions of rat aorta in Ca-free, Mg-free media were not accompanied by increased intracellular calcium concentration, [Ca2+](i), whereas such contractions in the presence of the divalent cations were correlated with rising [Ca2+](i) as assessed by fura-2. At the same time, calcium channel blockers, a modulator of Ca2+-binding proteins, and a modulator of actin polymerization, inhibited all types of K+ -induced contractions. Increasing the K+ in isotonic medium evoked a rise of (45)Ca2+ binding to the plasma membrane of freshly isolated aortic cells. Although Ca2+ -dependent events underlie the mechanism of K+ -induced vascular contractions in both the presence and absence of Ca2+, in contrast to the view that [Ca2+](i) is a key regulator of excitation-contraction coupling in smooth muscle, we suggest that the modulation of Mg2+ -dependent Ca2+ binding, probably within/at the L-type calcium channel by K+, is a trigger for aortic contraction. This Ca2+ binding may then activate actin-myosin interaction.

Cellular mechanisms involved in iso-osmotic high K+ solutions-induced contraction of the estrogen-primed rat myometrium

The aim of the present study was to investigate the mechanisms involved in the contraction evoked by iso-osmotic high K+ solutions in the estrogen-primed rat uterus. In Ca2+-containing solution, iso-osmotic addition of KCl (30, 60 or 90 mM K+) induced a rapid, phasic contraction followed by a prolonged sustained plateau (tonic component) of smaller amplitude. The KCl (60 mM)-induced contraction was unaffected by tetrodotoxin (3 microM), omega-conotoxin MVIIC (1 microM), GF 109203X (1 microM) or calphostin C (3 microM) but was markedly reduced by tissue treatment with neomycin (1 mM), mepacrine (10 microM) or U-73122 (10 microM). Nifedipine (0.01-0.1 microM) was significantly more effective as an inhibitor of the tonic component than of the phasic component. After 60 min incubation in Ca2+-free solution containing 3 mM EGTA, iso-osmotic KCl did not cause any increase in tension but potentiated contractions evoked by oxytocin (1 microM), sodium orthovanadate (160 micrM) or okadaic acid (20 microM) in these experimental conditions. In freshly dispersed myometrial cells maintained in Ca2+-containing solution and loaded with indo 1, iso-osmotic KCl (60 mM) caused a biphasic increase in the intracellular Ca2+ concentration ([Ca2+]i). In cells superfused for 60 min in Ca2+-free solution containing EGTA (1 mM), KCl did not increase [Ca2+]i. In Ca2+-containing solution, KCl (60 mM) produced a 76.0 +/- 16.2% increase in total [3H]inositol phosphates above basal levels and increased the intracellular levels of free arachidonic acid. These results suggest that, in the estrogen-primed rat uterus, iso-osmotic high K+ solutions, in addition to their well known effect on Ca2+ influx, activate other cellular processes leading to an increase in the Ca2+ sensitivity of the contractile machinery by a mechanism independent of extracellular Ca2+.

Signaling mechanisms underlying the vascular myogenic response

The vascular myogenic response refers to the acute reaction of a blood vessel to a change in transmural pressure. This response is critically important for the development of resting vascular tone, upon which other control mechanisms exert vasodilator and vasoconstrictor influences. The purpose of this review is to summarize and synthesize information regarding the cellular mechanism(s) underlying the myogenic response in blood vessels, with particular emphasis on arterioles. When necessary, experiments performed on larger blood vessels, visceral smooth muscle, and even striated muscle are cited. Mechanical aspects of myogenic behavior are discussed first, followed by electromechanical coupling mechanisms. Next, mechanotransduction by membrane-bound enzymes and involvement of second messengers, including calcium, are discussed. After this, the roles of the extracellular matrix, integrins, and the smooth muscle cytoskeleton are reviewed, with emphasis on short-term signaling mechanisms. Finally, suggestions are offered for possible future studies.

Effects of the potassium channel openers KRN4884 and levcromakalim on the contraction of rat aorta induced by A23187, compared with nifedipine

We examined the different vasodilatory effects of the K+ channel openers levcromakalim and 5-amino-N- [2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarboxamidine (KRN4884), and the Ca2+ channel blocker nifedipine in the rat aorta. KRN 4884 (10(-10)-10(-5) M) and nifedipine (10(-10)-10(-5) M) produced concentration-dependent relaxation in the rat aorta precontracted by 25 mM KCl. The K+ channel blocker glibenclamide (1 microM) inhibited the relaxation induced by KRN4884 but did not influence nifedipine-induced relaxation. KRN4884 had almost no effect on contraction induced by 80 mM KCl, whereas nifedipine completely relaxed the muscle precontracted by 80 mM KCl, whereas nifedipine completely relaxed the muscle precontracted by 80 mM KCl. These results indicate that KRN4884 is a K+ channel opener. We investigated the relaxant effects of KRN4884 (10(-10)-10(-5) M), levcromakalim (10(-9)-10(-5) M) and nifedipine (10(-9)-10(-5) M) on A23187 (1 microM)-induced contraction. KRN4884 and levcromakalim had a potent relaxant effect but nifedipine only a weak effect on the smooth muscle contracted by A23187. Glibenclamide (1 microM) inhibited the relaxation induced by KRN4884 and levcromakalim, but did not influence the nifedipine-induced relaxation. KRN4884 (1 microM) produced a larger relaxation of A23187-induced contraction but had little effect on the increase in intracellular [Ca2+] induced by A23187. These results suggest that KRN4884 is a specific K+ channel opener and its vasodilating mechanisms involve not only deactivation of Ca2+ channels but also a decrease in the Ca2+ sensitivity of contractile elements.

Endothelin-3 activates a voltage-gated Ca channel via a pertussis toxin sensitive mechanism leading to dopamine release from PC12 cells

We have investigated the role of endothelin-3 (ET-3) in the stimulus-secretion coupling mechanism in rat pheochromocytoma PC12 cells. ET-3 (10-100 nM) evoked both dopamine (DA) release and an increase in intracellular Ca2+ concentration ([Ca]i). The ET-3-evoked DA release was partially inhibited by pretreatment with pertussis toxin (PTX; 2 ng/ml, 20 h). The release was also attenuated by the voltage-gated Ca channel (VGC) blockers Cd2+ (300 microM) or nicardipine (30 microM) and was completely abolished when external Ca2+ was removed. ET-3-evoked [Ca]i increase was attenuated by the application of these VGC blockers and by pretreatment with PTX, and was abolished by removal of extracellular Ca2+. Removal of external Na+ had no effect on either response. In light of these findings, we conclude that ET-3 evokes both DA release and an increase in [Ca]i by a mechanism which involves the activation of PTX-sensitive VGCs and the resultant influx of Ca2+.

Do the K+ channel openers relax smooth muscle by opening K+ channels?

During the past decade, a group of chemically heterogeneous compounds known as the K+ channel openers has emerged. These compounds open a certain class of K+ channels (ATP-sensitive K+ channels) in the sarcolemma of vascular smooth muscle cells, which leads to hyperpolarization of the cell membrane and relaxation of the tissue. The mechanisms by which hyperpolarization affects smooth muscle contraction and contractility can thus be examined. Hyperpolarization induced by these K+ channel openers prevents Ca2+ entry through voltage-operated Ca2+ channels. Surprisingly, and by mechanisms not yet defined, hyperpolarization of the cell also reduces agonist-induced accumulation of inositol 1,4,5-trisphosphate (and consequently, Ca2+ mobilization from intracellular stores), and the Ca2+ sensitivity of the contractile apparatus. In addition, recent evidence reviewed here by Ulrich Quast suggests that the K+ channel openers possess further mechanisms of vasorelaxation not linked to the opening of plasmalemmal K+ channels.