Calcium influx and protein kinase C activation involved in uterine vasoconstriction in guinea pigs

Uteroplacental Circulation in Normal Pregnancy and Preeclampsia: Functional Adaptation and Maladaptation

Uteroplacental blood flow increases as pregnancy advances. Adequate supply of nutrients and oxygen carried by uteroplacental blood flow is essential for the well-being of the mother and growth/development of the fetus. The uteroplacental hemodynamic change is accomplished primarily through uterine vascular adaptation, involving hormonal regulation of myogenic tone, vasoreactivity, release of vasoactive factors and others, in addition to the remodeling of spiral arteries. In preeclampsia, hormonal and angiogenic imbalance, proinflammatory cytokines and autoantibodies cause dysfunction of both endothelium and vascular smooth muscle cells of the uteroplacental vasculature. Consequently, the vascular dysfunction leads to increased vascular resistance and reduced blood flow in the uteroplacental circulation. In this article, the (mal)adaptation of uteroplacental vascular function in normal pregnancy and preeclampsia and underlying mechanisms are reviewed.

Possible mechanisms underlying pregnancy-induced changes in uterine artery endothelial function

The last 10 years has seen a dramatic increase in our understanding of the mechanisms underlying the pregnancy-specific adaptation in cardiovascular function in general and the dramatic changes that occur in uterine artery endothelium in particular to support the growing fetus. The importance of these changes is clear from a number of studies linking restriction of uterine blood flow (UBF) and/or endothelial dysfunction and clinical conditions such as intrauterine growth retardation (IUGR) and/or preeclampsia in both humans and animal models; these topics are covered only briefly here. The recent developments that prompts this review are twofold. The first is advances in an understanding of the cell signaling processes that regulate endothelial nitric oxide synthase (eNOS) in particular (Govers R and Rabelink TJ. Am J Physiol Renal Physiol 280: F193-F206, 2001). The second is the emerging picture that uterine artery (UA) endothelial cell production of nitric oxide (NO) as well as prostacyclin (PGI2) may be as much a consequence of cellular reprogramming at the level of cell signaling as due to tonic stimuli inducing changes in the level of expression of eNOS or the enzymes of the PGI2 biosynthetic pathway (cPLA2, COX-1, PGIS). In reviewing just how we came to this conclusion and outlining the implications of such a finding, we draw mostly on data from ovine or human studies, with reference to other species only where directly relevant.

Involvement of erythropoietin-induced cytosolic free calcium mobilization in activation of mitogen-activated protein kinase and DNA synthesis in vascular smooth muscle cells

BACKGROUND/OBJECTIVE

Human recombinant erythropoietin (rHuEPO) induces cytosolic free calcium ([Ca2+]i) mobilization, an activation of mitogen-activated protein (MAP) kinase and DNA synthesis in several tissues. We explored the mechanism of rHuEPO-induced [Ca2+]i mobilization and its role in the activation of MAP kinase and DNA synthesis in vascular smooth muscle cells (VSMC).

METHODS

[Ca2+]i concentrations were measured by fura-2. MAP kinase activation was analyzed using an immunocomplex kinase assay and Western blotting. DNA synthesis was measured as an incorporation of 5-bromo-2'-deoxyuridine.

RESULTS

Although addition of rHuEPO significantly increased [Ca2+]i, either in the presence or absence of extracellular Ca2+, the peak level and sustained elevation of [Ca2+]i were significantly reduced in the absence of extracellular Ca2+. Pretreatment with genistein completely blocked the elevation of [Ca2+]i in both conditions. Calphostin C and staurosporine did not completely block the elevation of [Ca2+]i. Staurosporine reduced its peak level in a dose-dependent manner, whereas calphostin C reduced its peak level at concentrations over 1 nmol/l in the presence of extracellular Ca2+. Similar results to those with staurosporine were observed with nifedipine. In the absence of extracellular Ca2+, their dose-dependent effects disappeared even though rHuEPO increased [Ca2+]i. rHuEPO activated MAP kinase and DNA synthesis, both of which were significantly suppressed by the chelation of intracellular Ca2+.

CONCLUSION

These findings suggest that rHuEPO increases [Ca2+]i by both Ca2+ influx and Ca2+ release from intracellular stores. Tyrosine phosphorylation is critical in the regulation of [Ca2+]i, but protein kinase C activation is important only in the regulation of Ca2+ influx. Dihydropyridine-sensitive L-type Ca2+ channels seem to be involved in rHuEPO-induced Ca2+ influx. In addition, increase of [Ca2+]i by rHuEPO stimulates MAP kinase activation and DNA synthesis in VSMC.

Neuropeptide Y potentiates noradrenaline-evoked vasoconstriction by an intracellular calcium-dependent mechanism

The potentiating effect of neuropeptide Y (NPY) was examined by testing the influence of putative inhibitors of calcium entry on the NPY-enhanced contractile response to noradrenaline in the guinea pig uterine artery. In order to examine the involvement of voltage sensitive calcium entry mechanisms we recorded the effect of noradrenaline and NPY on the membrane potential. NPY (100-300 nM) enhanced noradrenaline-evoked vasoconstriction. The potentiation by NPY was most prominent in low noradrenaline concentrations (30-300 nM) and the pD10 (-log molar concentration of agonist eliciting 10% of maximum contraction) value was increased from 6.43 +/- 0.07 to 6.97 +/- 0.11 (P < 0.001, n = 6). Inhibition of extracellular calcium influx shifted concentration-dependently to the right the concentration-response curve for noradrenaline but potentiation by NPY still remained. The intracellular calcium chelator quin-2 AM selectively abolished the NPY-induced enhancement of the contractile response to noradrenaline. In contrast, quin-2 AM (10-30 microM) had no inhibitory effect on the contractile response to noradrenaline per se. It is suggested that NPY initiates an intracellular calcium-sensitive mechanism which increase alpha-adrenoceptor sensitivity. This results in a significant increase of sarcoplasmic calcium and stronger contractile responses to noradrenaline.

Effect of extracellular Ca2+ on cholinergic, KCl and phorbol ester-mediated phosphoinositide turnover and guinea pig urinary bladder contraction

The effect of extracellular Ca2+ ([Ca2+]o) on cholinergic, KCl and phorbol ester-mediated detrusor contractions was related to phosphoinositide (PI) breakdown in guinea pig urinary bladder. Carbachol (1.0 mM) elicited a 20-fold increase in inositol phosphate (IP) accumulation both in presence and absence of [Ca2+]o yielding the same EC50 value (approximately 12 microM). In contrast, carbachol-induced detrusor contractions were reduced by 35% without [Ca2+]o, but maximal efficacy was restored with Ca2+ replenishment. In absence of [Ca2+]o, repeated cholinergic stimulation yielded contractions only if tissues were intermittently equilibrated in [Ca2+]o. High K+ and PDBu evoked [Ca2+]o-dependent contractions. Ca2+ channel antagonists and divalent metal cations inhibited high K+ more potently than carbachol-mediated contractions. Together, these findings suggest multiple sources of Ca2+ for urinary bladder contraction, where voltage-sensitive responses depend primarily on [Ca2+]o and PI-linked muscarinic responses involved Ca2+ mobilization from intracellular stores as well. Clinical agents used for the treatment of urinary incontinence inhibited both carbachol-induced PI turnover and muscle contraction with the same rank order of potency both in presence and absence of [Ca2+]o. These findings suggest that the cholinergic mechanism of action of these agents involves the PI-Ca2+ effector system.

Vasodilatory effects of nifedipine, methoxyverapamil, and sodium nitroprusside on contractile responses of the ewe uterine artery at term pregnancy

The differential inhibitory effect of the vasodilators on contractile responses to norepinephrine, serotonin, and potassium on isolated uterine artery ring segments from pregnant ewes within 2 weeks of term was quantified and correlated with the source of Ca++ for the vasoconstrictors producing the smooth muscle contraction. The contraction evoked by the vasoconstrictors was dependent on extracellular Ca++ and in agonist-induced contractions also on an intracellular pool of Ca++. Nifedipine effectively inhibited K(+)-induced (90 mmol/L) contractions (antagonist concentration to reduce the maximum contractile effect to the agonist to 50%, 1.95 +/- 0.9 x 10(-8) mol/L), whereas it was relatively ineffective in blocking norepinephrine-induced (10(-5) mol/L) or serotonin-induced (10(-5) mol/L) vasoconstriction (antagonist concentration to reduce the maximum contractile effect to the agonist to 50%, 1.38 +/- 0.4 x 10(-4) mol/L and 2.04 +/- 0.4 x 10(-5) mol/L, respectively). Methoxyverapamil (D-600) strongly inhibited serotonin-induced contractions (antagonist concentration to reduce the maximum contractile effect to the agonist to 50%, 3.3 +/- 0.3 x 10(-7) mol/L). The phasic rather than the tonic components of the serotonin- and norepinephrine-induced contractions were more effectively inhibited by D-600 (p less than 0.05). Sodium nitroprusside preferentially blocked (p less than 0.05) the sustained tonic components of norepinephrine- and serotonin-induced vasoconstrictions (antagonist concentration to reduce the maximum contractile effect to the agonist to 50%, 7.1 +/- 0.4 x 10(-7) mol/L and 8.2 +/- 0.6 x 10(-7) mol/L, respectively). On the basis of these findings it is concluded that D-600 and sodium nitroprusside are more effective than nifedipine in blocking contractile responses due to receptor stimulation, and therefore might be more effective in the treatment of hypertensive emergencies in which these amines might be implicated.