Mária Pintérová

Nifedipine-sensitive noradrenergic vasoconstriction is enhanced in spontaneously hypertensive rats: the influence of chronic captopril treatment

Aim:  The relationship between increased sympathetic tone and enhanced activity of L‐type voltage‐dependent Ca2+ channels (L‐VDCC) in spontaneously hypertensive rats (SHR) was studied using in vivo and in vitro approaches.

Role of nifedipine-sensitive sympathetic vasoconstriction in maintenance of high blood pressure in spontaneously hypertensive rats: effect of Gi-protein inactivation by pertussis toxin.

Background High blood pressure (BP) in spontaneously hypertensive rats (SHRs) is attributed to excessive activity of sympathetic nervous system (SNS) and relative nitric oxide deficiency. An important part of SNS hypertensive action is exerted by calcium influx through L-type of voltage-dependent calcium channels (L-VDCC). The overexpression of pertussis toxin (PTX)-sensitive inhibitory G-proteins (Gi) participating in the development and maintenance of high BP in SHRs suggested us to study Gi-protein involvement in the pathway through which noradrenergic vasoconstriction and calcium influx can be coupled. Method The participation of main vasoactive systems (angiotensin II, norepinephrine, nitric oxide) in BP maintenance was investigated in conscious SHR and WKY rats (half of them being pretreated with PTX, 10 μg/kg i.v., 48 h before the experiment). To evaluate the contribution of Gi-proteins and L-VDCC to vasoconstriction induced by exogenous norepinephrine, dose–response curves were determined before and after acute nifedipine administration. Results PTX pretreatment of SHRs significantly decreased BP and reduced sympathetic vasoconstriction, which was partially substituted by enhanced angiotensin II-dependent vasoconstriction. PTX pretreatment also reduced nitric oxide-dependent vasodilation in both rat strains. PTX pretreatment of SHRs decreased BP component sensitive to acute blockade of calcium entry by nifedipine. In both strains, PTX pretreatment as well as acute nifedipine administration caused substantial rightward shift of norepinephrine dose–response curves (without additive effects of both treatments). Conclusion The enhanced contribution of SNS to hypertension maintenance in SHRs is mediated by Gi-protein-coupled pathway controlling calcium influx through L-VDCC.

Vasodilator efficiency of endogenous prostanoids, Ca²⁺-activated K⁺ channels and nitric oxide in rats with spontaneous, salt-dependent or NO-deficient hypertension.

Hypertension is associated with the imbalance of vasoconstrictor and vasodilator systems. Vasodilation is usually evaluated in isolated blood vessels, but except for nitric oxide (NO), relatively little attention is given to the in vivo efficiency of particular vasodilator mechanisms. The aim of our study was to evaluate the contribution of endogenous vasodilator prostanoids, Ca2+-activated K+ channels and NO to blood pressure (BP) maintenance in rats with three different forms of experimental hypertension. Both principal vasopressor systems (the renin–angiotensin system and the sympathetic nervous system) were blocked by captopril and pentolinium in conscious spontaneously hypertensive rats (SHRs), Dahl salt-hypertensive (DS-HS) rats and rats with NO-deficient hypertension, as well as in their normotensive controls. Thereafter, we monitored BP changes in rats subjected to either a sequential or an isolated blockade of prostanoid synthesis by the non-selective cyclooxygenase inhibitor, indomethacin, of Ca2+-activated K+ channels by tetraethylammonium and of NO formation by NG-nitro-L-arginine methyl ester. All three forms of experimental hypertension were characterized by augmented sympathetic vasoconstriction. The vasodilatation exerted by endogenous prostanoids and Ca2+-activated K+ channels was enhanced in all forms of hypertension, almost proportionally to BP elevation. On the contrary, NO-dependent vasodilatation was not enhanced in any form of experimental hypertension, and there was a severe relative NO deficiency in both, SHRs and DS-HS rats. In conclusion, our data suggested that there is a compensatory activation of vasodilator prostanoids and Ca2+-activated K+ channels in rats with experimental hypertension, whereas NO-dependent vasodilatation is not augmented. Thus, the overall activity of vasodilator systems failed to compensate for augmented sympathetic vasoconstriction in hypertensive animals.

Ca2+ sensitization and Ca2+ entry in the control of blood pressure and adrenergic vasoconstriction in conscious Wistar-Kyoto and spontaneously hypertensive rats.

Background: Calcium entry through nifedipine-sensitive L-type voltage-dependent calcium channels (L-VDCC) is augmented in spontaneously hypertensive rats (SHR) characterized by enhanced sympathetic vasoconstriction. However, the changes of calcium sensitization mediated by RhoA/Rho kinase pathway are less understood. Methods and results: The participation of calcium entry and calcium sensitization in the control of blood pressure (BP) and vascular contraction was studied in SHR and normotensive Wistar–Kyoto (WKY) rats. The acute administration of fasudil (Rho kinase inhibitor) caused BP decrease which lasted longer in SHR. Fasudil also attenuated adrenergic contraction in femoral or mesenteric arteries of WKY and SHR. BP reduction elicited by fasudil in WKY was more pronounced than that induced by L-VDCC blocker nifedipine (−33 ± 2 vs. −15 ± 3% of baseline BP, P < 0.001), whereas both inhibitors were similarly effective in SHR (−36 ± 4 vs. −41 ± 2%). Fasudil pretreatment also attenuated BP elevation elicited by L-VDCC agonist BAY K8644 more in WKY than in SHR (−63 ± 4 vs. −42 ± 5%, P < 0.001), indicating reduced calcium sensitization in SHR. Moreover, fasudil pretreatment shifted norepinephrine dose–response curves to the right more in WKY than in SHR. The additional nifedipine pretreatment shifted these curves further to the right but this shift was more pronounced in SHR than in WKY. Thus adrenergic vasoconstriction is more dependent on L-VDCC in SHR and on RhoA/Rho kinase pathway in WKY rats. Conclusion: Ca2+ sensitization mediated by RhoA/Rho kinase pathway is attenuated in SHR compared with normotensive WKY rats. This might be a part of the compensation for enhanced Ca2+ entry through L-VDCC in genetic hypertension.

INVOLVEMENT OF LARGE CONDUCTANCE POTASSIUM CHANNELS IN THE VASODILATION INDUCED BY OVERPRODUCTION OF CYCLIC AMP: IN VIVO STUDY IN RATS WITH GENETIC HYPERTENSION: PP.29.148

Objective: We have shown that cyclic nucleotides (cAMP and cGMP) are involved in the control of Ca2+ influx into the vascular smooth muscle cells by closing L-VDCC (L-type of voltage-dependent calcium channels). Overproduction of cAMP induced by in vivo inactivation of inhibitory G-proteins as well as by activation of beta-adrenoceptors with isoprenaline infusion diminished Ca2+ influx through L-VDCC elicited by the absence of cGMP occurring after acute nitric oxide (NO) synthase blockade (L-NAME). One of the possible pathway by which cAMP inhibits these channels could be the activation of Ca2+-dependent potassium channels (BKCa) leading to the membrane hyperpolarization and thus closing of L-VDCC. Design and Method: To confirm this hypothesis we investigated whether inhibition of BKCa channels can modify the ability of cAMP overproduction to prevent blood pressure (BP) rise induced by acute NO depletion using conscious normotensive (WKY) and hypertensive (SHR) rats. Enhanced production of cAMP was induced by beta-adrenoceptor stimulation using isoprenaline infusion (100 ng/kg/min i.v.), which was followed by inhibition of NO formation with L-NAME (30 mg/kg i.v.). To deternine the role of BKCa continuous infusion of BKCa blocker (TEA, 100 mg/kg/h i.v.) was started before isoprenaline infusion. Results: Isoprenaline-induced overproduction of cAMP caused significant BP fall in both rat strains (substantially more in SHR) and prevented BP rises induced by acute inhibition of NO formation. This prevention of L-NAME-induced BP rise was diminished by the blockade of BKCa with TEA. Interestingly, the completely restored BP response to L-NAME injection was observed only in SHR while in WKY the restoration was only partial. Conclusions: Major part of the inhibitory effect of cyclic AMP on calcium entry through L-VDCC in genetically hypertensive rats is mediated by the activation of large conductance calcium-activated potassium channels. However, the involvement of these channels in cAMP-induced vasodilation in normotensive rats was largely attenuated.

GENDER DIFFERENCE IN BLOOD PRESSURE OF SPONTANEOUSLY HYPERTENSIVE RATS IS NOT MEDIATED BY THE CHANGES IN CALCIUM ENTRY THROUGH VOLTAGE-DEPENDENT CALCIUM CHANNELS OF L TYPE: PP.29.142

Objective: It is well known that blood pressure (BP) is higher in male than female rats in most forms of genetic hypertension. This difference is usually ascribed to prohypertensive effects of androgens and/or antihypertensive action of estrogens because BP difference can be attenuated by castration. Several reports suggested that androgens increase the expression of alpha subunits of L type of voltage-dependent calcium channels (L-VDCC). These channels are involved in the control of vascular tone and agonist-induced vasoconstriction in both normotensive and hypertensive rats. Design and Method: To investigate the role of L-VDCC in gender-dependent BP control in spontaneously hypertensive rats (SHR), we have compared BP response to acute nifedipine injections (0.05 – 0.4 mg/kg i.v.) in intact or neonatally castrated male and female SHR. Results: Although BP was significantly elevated in male SHR compared to female SHR, there was no difference in the involvement of L-VDCC measured as BP reduction elicited by nifedipine administration. Neonatal castration abolished gender difference in BP of SHR, but again without significant alterations of nifedipine-sensitive BP component. Conclusions: The gender difference in blood pressure of spontaneously hypertensive rats can not be ascribed to different calcium entry through voltage-dependent calcium channels of L type susceptible to the inhibition by nifedipine.