Mechanism of calcium activation in vascular smooth muscle

Pregnancy-induced Cardiovascular Pathologies: Importance of Structural Components and Lipids

Pregnancy leads to adaptations for maternal and fetal energy needs. The cardiovascular system bears the brunt of the adaptations as the heart and vessels enable nutrient supply to maternal organs facilitated by the placenta to the fetus. The components of the cardiovascular system are critical in the balance between maternal homeostatic and fetus driven homeorhetic regulation. Since lipids intersect maternal cardiovascular function and fetal needs with growth and in stress, factors affecting lipid deposition and mobilization impact risk outcomes. Here, the cardiovascular components and functional derangements associated with cardiovascular pathology in pregnancy, vis-à-vis lipid deposition, mobilization and maternal and/or cardiac and fetal energy needs are detailed. Most reports on the components and associated pathology in pregnancy, are on derangements affecting the extracellular matrix and epicardial fat, followed by the endothelium, vascular smooth muscle, pericytes and myocytes. Targeted studies on all cardiovascular components and pathological outcomes in pregnancy will enhance targeted interventions.

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Pregnancy-associated adaptations in [Ca2+]i-dependent and Ca2+ sensitization mechanisms of venous contraction: implications in pregnancy-related venous disorders

Pregnancy is associated with significant adaptations in the maternal hemodynamics and arterial circulation, but the changes in the venous mechanisms during pregnancy are less clear. We hypothesized that pregnancy is associated with alterations in venous function, intracellular free Ca(2+) concentration ([Ca(2+)]i), and Ca(2+)-dependent mechanisms of venous contraction. Circular segments of inferior vena cava (IVC) from virgin and late pregnant (Preg, day 19) Sprague-Dawley rats were suspended between two hooks, labeled with fura-2, and placed in a cuvet inside a spectrofluorometer for simultaneous measurement of contraction and [Ca(2+)]i (fura-2 340/380 ratio). KCl (96 mM), which stimulates Ca(2+) influx, caused less contraction (35.6 ± 6.3 vs. 92.6 ± 19.9 mg/mg tissue) and smaller increases in [Ca(2+)]i (1.67 ± 0.12 vs. 2.19 ± 0.11) in Preg vs. virgin rat IVC. The α-adrenergic receptor agonist phenylephrine (Phe; 10(-5) M) caused less contraction (23.8 ± 3.4 vs. 70.9 ± 12.9 mg/mg tissue) and comparable increases in [Ca(2+)]i (1.76 ± 0.10 vs. 1.89 ± 0.08) in Preg vs. virgin rat IVC. At increasing extracellular Ca(2+) concentrations ([Ca(2+)]e) (0.1, 0.3, 0.6, 1, and 2.5 mM), KCl and Phe induced [Ca(2+)]e-contraction and [Ca(2+)]e-[Ca(2+)]i curves that were reduced in Preg vs. virgin IVC, supporting reduced Ca(2+) entry mechanisms. The [Ca(2+)]e-contraction and [Ca(2+)]e-[Ca(2+)]i curves were used to construct the [Ca(2+)]i-contraction relationship. Despite reduced contraction and [Ca(2+)]i in Preg IVC, the Phe-induced [Ca(2+)]i-contraction relationship was greater than that of KCl and was enhanced in Preg vs. virgin IVC, suggesting parallel activation of Ca(2+)-sensitization pathways. The Ca(2+) channel blocker diltiazem, protein kinase C (PKC) inhibitor GF-109203X, and Rho-kinase (ROCK) inhibitor Y27632 inhibited KCl- and Phe-induced contraction and abolished the shift in the Phe [Ca(2+)]i-contraction relationship in Preg IVC, suggesting an interplay between the decrease in Ca(2+) influx and possible compensatory activation of PKC- and ROCK-mediated Ca(2+)-sensitization pathways. The reduced [Ca(2+)]i and [Ca(2+)]i-dependent contraction in Preg rat IVC, despite the parallel rescue activation of Ca(2+)-sensitization pathways, suggests that the observed reduction in [Ca(2+)]i-dependent contraction mechanisms is likely underestimated, and that the veins without the rescue Ca(2+)-sensitization pathways could be even more prone to dilation during pregnancy. These pregnancy-associated reductions in Ca(2+) entry-dependent mechanisms of venous contraction, if occurring in human lower extremity veins and if not adequately compensated by Ca(2+)-sensitization pathways, may play a role in pregnancy-related venous disorders.

Rho-kinase, a common final path of various contractile bladder and ureter stimuli

Normal urinary bladder function is based on the proper contraction and relaxation of smooth muscle (SM), which constitutes the majority of the bladder wall. The contraction and relaxation of all SM involves a phosphorylation-dephosphorylation pathway involving the enzymes smooth muscle myosin light chain kinase (SMMLCK) and smooth muscle myosin light chain phosphatase (SMMLCP), respectively. Although originally thought to function just as a passive opposition to SMMLCK-driven SM contraction, it is now clear that SMMLCP activity is under an extremely complex molecular regulation via which SMMLCP inhibition can induce "calcium sensitization." This review provides a thorough summary of the literature regarding the molecular regulation of the SMMLCP with a focus on one of its major inhibitory pathways that is RhoA/Rho-kinase (ROK) including its activation pathways, effector molecules, and its roles in various pathological conditions associated with bladder dysfunction. Newly emerging roles of ROK outside of SM contractility are also discussed. It is concluded that the RhoA/ROK pathway is critical for the maintenance of basal SM tone of the urinary bladder and serves as a common final pathway of various contractile stimuli in rabbits, rats, mice, and pigs as well as humans. In addition, this pathway is upregulated in response to a number of pathological conditions associated with bladder SM dysfunction. Similarly, RhoA/Rho-kinase signaling is essential for normal ureteral function and development and is upregulated in response to ureteral outlet obstruction. In addition to its critical role in bladder SM function, a role of ROK in the urothelium is also beginning to emerge as well as roles for ROK in bladder infection and invasion and metastasis of bladder cancer.

Sex-related decrease in [Ca2+]i signaling and Ca2+-dependent contraction in inferior vena cava of female rat

Sex differences in the incidence of varicose veins have been suggested; however, the venous mechanisms involved are unclear. We hypothesized sex-related differences in venous function and underlying distinctions in intracellular free calcium, [Ca(2+)](i), signaling and Ca(2+)-dependent mechanisms of venous contraction. Circular segments of inferior vena cava (IVC) from male and female Sprague-Dawley rats were suspended between two hooks, labeled with fura-2, and placed in a cuvet inside a spectrofluorometer for simultaneous measurement of isometric contraction and the 340/380 fluorescence ratio (indicative of [Ca(2+)](i)). In male IVC, phenylephrine (PHE; 10(-5) M) caused significant increase in contraction and [Ca(2+)](i). In female IVC, PHE-induced contraction was significantly reduced, but [Ca(2+)](i) did not differ significantly from males. Membrane depolarization by KCl (96 mM), which stimulates Ca(2+) influx, caused parallel increases in contraction and [Ca(2+)](i) in male IVC, and the KCl-induced contraction was significantly reduced in parallel with [Ca(2+)](i) in female IVC. In male IVC stimulated with 0 Ca(2+) KCl solution, the addition of increasing concentrations of extracellular Ca(2+) ([Ca(2+)](e)) (0.1, 0.3, 0.6, 1, and 2.5 mM) caused stepwise increases in contraction and [Ca(2+)](i), and both the KCl-induced [Ca(2+)](e)-contraction curve and the [Ca(2+)](e)-[Ca(2+)](i) curve were reduced in female IVC, suggesting reduced Ca(2+) entry via voltage-gated channels. The PHE-induced [Ca(2+)](e)-contraction curve was significantly reduced in females, but the [Ca(2+)](e)-[Ca(2+)](i) curve was similar in female and male IVC, suggesting the involvement of other mechanisms in addition to Ca(2+) entry. The [Ca(2+)](e)-contraction and [Ca(2+)](e)-[Ca(2+)](i) curves were used to construct the [Ca(2+)](i)-contraction relationship. The KCl-induced [Ca(2+)](i)-contraction relationship was superimposed in male and female IVC. In contrast, the PHE-induced [Ca(2+)](i)-contraction relationship was reduced and located to the right in female compared with male IVC, suggesting reduced [Ca(2+)](i) sensitivity of the venous contractile myofilaments. The reduced contraction, [Ca(2+)](i), and [Ca(2+)](i) sensitivity in female veins render them more prone to dilation. These sex-specific reductions in venous function, if they also occur in human veins, may play a role in the greater incidence of varicose veins in females.

The mechanism of phenylephrine-mediated [Ca(2+)](i) oscillations underlying tonic contraction in the rabbit inferior vena cava

1. We characterized the mechanisms in vascular smooth muscle cells (VSMCs) that produce asynchronous, wave-like Ca(2+) oscillations in response to phenylephrine (PE). Confocal imaging was used to observe [Ca(2+)](i) in individual VSMCs of intact inferior vena cava (IVC) from rabbits. 2. It was found that the Ca(2+) waves were initiated by Ca(2+) release from the sarcoplasmic reticulum (SR) via inositol 1,4,5-trisphosphate-sensitive SR Ca(2+) release channels (IP(3)R channels) and that refilling of the SR Ca(2+) store through the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase (SERCA) was required for maintained generation of the repetitive Ca(2+) waves. 3. Blockade of L-type voltage-gated Ca(2+) channels (L-type VGCCs) with nifedipine reduced the frequency of PE-stimulated [Ca(2+)](i) oscillations, while additional blockade of receptor-operated channels/store-operated channels (ROCs/SOCs) with SKF96365 abolished the remaining oscillations. Parallel force measurements showed that nifedipine inhibited PE-induced tonic contraction by 27 % while SKF96365 abolished it. This indicates that stimulated Ca(2+) entry refills the SR to support the recurrent waves of SR Ca(2+) release and that both L-type VGCCs and ROCs/SOCs contribute to this process. 4. Application of the Na(+)-Ca(2+) exchanger (NCX) inhibitors 2',4'-dichlorobenzamil (forward- and reverse-mode inhibitor) and KB-R7943 (reverse-mode inhibitor) completely abolished the nifedipine-resistant component of [Ca(2+)](i) oscillations and markedly reduced PE-induced tone. 5. Thus, we conclude that each Ca(2+) wave depends on initial SR Ca(2+) release via IP(3)R channels followed by SR Ca(2+) refilling through SERCA. Na(+) entry through ROCs/SOCs facilitates Ca(2+) entry through the NCX operating in the reverse mode, which refills the SR and maintains PE-induced [Ca(2+)](i) oscillations. In addition some Ca(2+) entry through L-type VGCCs and ROCs/SOCs serves to modulate the frequency of the oscillations and the magnitude of force development.

The role of nitric oxide in the reduction of protein kinase C-induced contractile response in aortae from rats with portal hypertension

BACKGROUND/AIMS

Protein kinase C plays a role in the regulation of vascular cell contraction but its activity may be reduced by nitric oxide. In portal hypertension, the exact mechanism by which nitric oxide induces vascular hyporeactivity to vasoconstrictors is unclear. The aim of this study was to investigate the role of the interaction of nitric oxide and protein kinase C in the vascular reactivity in isolated aortae from portal vein-stenosed rats.

METHODS/RESULTS

The contractile response to phorbol 12,13-dibutyrate, a protein kinase C activator, was significantly reduced in portal vein-stenosed aortae compared to sham-operated aortae. Preincubation with N-nitro-L-arginine or endothelium removal enhanced the response to phorbol 12,13-dibutyrate. The hyporesponsiveness to phorbol 12,13-dibutyrate in portal vein-stenosed rat aortae was only corrected after endothelium removal. The time course of contractions induced by phorbol 12,13-dibutyrate showed that the contraction was maintained for 2 h in sham-operated aortae and decreased to baseline in portal vein-stenosed rat aortae. This decrease was inhibited by N-nitro-L-arginine preincubation or endothelium removal. Protein kinase C downregulation caused a more marked reduction of phenylephrine-induced contraction in portal vein-stenosed aortae than in sham-operated aortae. The time course of total nitric oxide synthase activity in the presence of phorbol 12,13-dibutyrate showed a decrease in nitric oxide synthase activity after 30 min in both groups. Nitric oxide synthase activity remained stable for 120 min in sham-operated aortae but returned to basal level in portal vein-stenosed aortae.

CONCLUSIONS

Hyporeactivity to vasoconstrictors in portal vein-stenosed rat aortae may be due, in part, to a decrease in protein kinase C activation caused by nitric oxide overproduction.

The compliance of the porcine pulmonary artery depends on pressure and heart rate

1. The influence of mean pulmonary arterial pressure (mean Ppa) on dynamic (Cd) and pseudo-static compliance (Cps) of the pulmonary artery was studied at a constant and a changing heart rate. Cd is the change in cross-sectional area (CSA) relative to the change in Ppa throughout a heart cycle. Cps is the change in mean CSA relative to the change in mean Ppa. If Cd is known, pulmonary blood flow can be computed from the Ppa using a windkessel model. We investigated whether Cps can be interchanged with Cd. 2. In nine anaesthetized pigs, we determined the mean CSA and Cd of the pulmonary artery at various Ppa levels, ranging from approximately 30 to 10 mmHg, established by bleeding. Two series of measurements were carried out, one series at a spontaneously changing heart rate (n = 9) and one series at a constant heart rate (n = 6). To determine CSA a conductance method was used. 3. Cps depended on pressure. The mean CSA versus mean Ppa curves were sigmoid and steepest in the series with the increasing heart rate (established by bleeding). The CSA versus Ppa loop during a heart cycle, giving Cd, was approximately linear and almost closed. The Cd versus mean Ppa relationship was bell shaped. Its width was smaller if the heart rate increased during the series of measurements. The pressure, where Cd was maximum, was higher at higher heart rates. Furthermore, the maximum Cd was not affected by the heart rate. 4. Because the pulmonary artery constricts with increasing heart rate, Cps will be overestimated during procedures where heart rate increases. Cd should be determined on a beat-to-beat basis to calculate flow because it changes with mean pulmonary arterial pressure and heart rate.

Action of probucol in arteries from normal and hypercholesterolaemic rabbits

1. The effect of probucol on the vascular reactivity of different arteries isolated from rabbits was studied as well as its effects on the development of atherosclerosis in a cholesterol-fed rabbit model. 2. Probucol 10(-6)-5 x 10(-4)M produced a concentration-dependent inhibition of the contractile responses induced by KCI (80 mM), the sequence for the IC50 was: mesenteric artery (5th branch, 4.8 +/- 2.6 x 10(-5) M) > aorta (8.2 +/- 2.3 x 10(-5) M) > femoral artery (> 5 x 10(-4) M). The response to noradrenaline was: mesenteric artery (5th branch, 4.2 +/- 1.3 x 10(-5) M) > aorta (3.2 +/- 3.0 x 10(-4) M) > femoral (> 5 x 10(-4) M). 3. In the aorta, probucol (10(-5)-10(-4) M) shifted the concentration-response curves to Ca2+ downward and to the right. 4. Probucol at 5 x 10(-5) M and 5 x 10(-4) M showed a reduction in the 45Ca2+ uptake in resting, non-stimulated aortic rings as well as the uptake induced by both noradrenaline 10(-6) M and KCI 80 mM. 5. In experiments in vivo, probucol did not affect lipid profiles; however, drug-treatment significantly decreased the cholesterol content of aortic tissue and the extent of intimal surface covered with atherosclerotic lesions. 6. The vascular reactivity was recovered in femoral arteries from rabbits on the atherogenic diet plus probucol. 7. It is concluded that the effect of probucol in vascular smooth muscle can be attributed to an inhibition of Ca2+ entry through both potential- and receptor-operated pathways. Moreover our findings suggest that the effects of probucol on movement of calcium in vascular smooth muscle may play an important role in the mechanism of antiatherogenic properties of this drug.

Regulation of cytosolic calcium levels in vascular smooth muscle

Ca2+ plays an important role in the contraction of skeletal, cardiac, and smooth muscle, as well as in a number of important processes, such as secretion and neuronal activity. In this review, I focus on the various mechanisms by which cytosolic Ca2+ concentration is regulated in vascular smooth muscle, in the resting state and during activation. Particular attention is paid to the calcium pumps of the plasmalemma and the sarcoplasmic reticulum, to the inositol 1,4,5-trisphosphate- and ryanodine-sensitive calcium channels of the sarcoplasmic reticulum, and to voltage-dependent and voltage-independent calcium channels of the plasmalemma.

Comparative study of elgodipine and nisoldipine on the contractile responses of various isolated blood vessels

The effects of elgodipine, a new dihydropyridine derivative, were compared to those of nisoldipine on contractile responses in various isolated artery rings and on mechanical activity in portal vein segments. Arteries used were: rabbit aorta, mesenteric (fifth branch), femoral and basilar, and sheep coronary arteries. Elgodipine and nisoldipine (10(-16)-3 x 10(-6) M) produced a concentration-dependent inhibition of the contractile responses induced by high K+ (80 mM), 5-hydroxytryptamine (10(-5) M) or noradrenaline (10(-6) M or 10(-4) M) in all the arteries studied. The inhibitory effect of elgodipine was greater in mesenteric resistance vessels (IC50 = 8.0 +/- 2.1 x 10(-12) M and 2.0 +/- 0.5 x 10(-13) M for the depression of high K(+)- and agonist-induced contraction, respectively), and in coronary arteries (IC50 = 2.6 +/- 0.3 x 10(-10) M and 9.0 +/- 1.4 x 10(-8) M for the inhibition of high K(+)- and agonist-induced contraction, respectively). In addition, the action of elgodipine in peripheral resistance vessels and in the coronary artery was more prominent than in aorta or femoral arteries, and this tissue selectivity was more apparent for elgodipine than for nisoldipine. In rat portal vein elgodipine (IC50 = 6.5 +/- 0.9 x 10(-8) M) and nisoldipine (IC50 = 8.5 +/- 1.3 x 10(-8) M) reduced in a concentration-dependent manner the development of mechanical activity. Furthermore, contractile responses produced by the addition of Ca2+ (1-5 mM) to Ca(2+)-free high K+ solution were also concentration dependently inhibited by elgodipine. However, elgodipine did not modify noradrenaline-induced contractions attributed to intracellular Ca2+ release. The results of this study indicate that elgodipine has potent vasodilator properties and vascular selectivity. The mechanisms through which elgodipine relaxes vascular smooth fibres seem to be related to its ability to inhibit the entry of extracellular Ca2+ into the cell.

The concentration of magnesium in erythrocytes in female patients with primary Raynaud's phenomenon; fluctuation with the time of year

The magnesium (Mg) concentration was measured in erythrocytes of 30 women with pronounced primary Raynaud's phenomenon (PRP) and of 33 age-matched healthy control women. The measurements were made at three different times, six months apart. The mean Mg concentration in the PRP group and the control group differed significantly at each measurement. The values for the two groups were, respectively, 1.64 +/- 0.21 and 2.02 +/- 0.21 mmol/L (P < 0.001) in February, 1988; 2.16 +/- 0.21 and 1.85 +/- 0.21 mmol/L (P < 0.001) in August, 1988; and 2.01 +/- 0.21 and 2.37 +/- 0.20 mmol/L (P < 0.001) in January, 1989. The erythrocyte Mg level varied significantly with the time of year in both groups. In the PRP group the mean value was significantly higher in August than in the two winter months, but in the control group it was significantly lower in August. The authors conclude that women with PRP have a significantly lower magnesium concentration in erythrocytes during winter than the healthy controls and that this concentration varied with the season of the year in both groups.

Effects of indapamide on contractile responses and 45Ca2+ movements in various isolated blood vessels

The effects of indapamide on contractile responses in various isolated artery rings and on spontaneous mechanical activity in portal vein segments were investigated. Arteries used were: rabbit aorta, mesenteric (fifth branch), femoral and basilar, and sheep coronary arteries. 45Ca2+ uptake was also analysed in rabbit mesenteric arteries. Indapamide (10(-10)-3 x 10(-4) M) produced a concentration-dependent inhibition of the contractile responses induced by high K+ (80 mM), 5-hydroxytryptamine (10(-5) M), and noradrenaline (10(-6) or 10(-4) M) in all the arteries studied. The inhibitory effect was greater in mesenteric (fifth branch) the IC50 values being 9.2 +/- 3.0 x 10(-6) and 5.5 +/- 4.0 x 10(-8) M for depression of high K(+)- and agonist-induced contractions, respectively. Indapamide inhibited in a concentration-dependent manner the contractile responses elicited by the addition of Ca2+ (1-5 mM) to Ca(2+)-free high K+ solution as well as the spontaneous mechanical activity of rat portal vein. Indapamide also reduced the 45Ca2+ uptake in rabbit mesenteric arteries stimulated by noradrenaline (10(-4) M) or by high K+ (80 mM) without affecting the Ca2+ influx in resting tissues. Our results indicate that indapamide blocked both depolarization-and noradrenaline-induced Ca2+ influx while it did not modify passive Ca2+ entry. Peripheral resistance vessels were demonstrated to be the arteries most sensitive to indapamide vascular effects.

Depressor effect of intraventricular administration of calcium on spontaneously hypertensive rats

The role of central Ca2+ in the regulation of blood pressure (BP) was investigated in conscious spontaneously hypertensive (SHR) and Wistar-Kyoto rats (WKY). Ten microliters of a high Ca2+ solution (Ca2+: 32.6 mM) administered intracerebroventricularly (i.c.v.) decreased the mean arterial pressure (MAP) for more than 20 min in SHR (n = 7, P < 0.005), while no change of MAP was observed in the WKY (n = 6). This depressor response to Ca2+ i.c.v. was dose-dependent at Ca2+ concentrations between 16.3 and 65.2 mM. We also investigated the effect of high Ca2+ i.c.v. in SHR after pretreatment with Ca2+ channel blockers, diltiazem (60 micrograms/10 microliters) or nisoldipine (4, 8, 16 and 32 micrograms/10 microliters), administered i.c.v., the autonomic ganglion blocker, hexamethonium (50 mg/kg), administered i.v. and alpha-methyl-p-tyrosine (100 and 400 micrograms/10 microliters) delivered i.c.v. Pretreatment with i.c.v. diltiazem (n = 8) or nisoldipine (n = 5 for 8 micrograms, n = 6 for 4, 16, 32 micrograms) abolished and/or blunted the decrease of MAP due to high Ca2+. Hexamethonium administered i.v. (n = 6) also canceled the depressor action of i.c.v. Ca2+. Pretreatment with 100 micrograms of i.c.v. alpha-methyl-p-tyrosine could not prevent the depressor action of i.c.v. Ca2+; however, 400 micrograms of alpha-methyl-p-tyrosine administered i.c.v. abolished the effect of i.c.v. Ca2+. Furthermore Ca2+ channel blockers administered i.c.v. in themselves increased MAP in SHR (P < 0.05). These results suggest that central Ca2+ is involved in the central regulation of BP in SHR. This effect may be mediated through changes in sympathetic activity.

Influence of the endothelium on Ca2+ dependency of angiotensin II-induced contractions of rat aortic rings

Endothelium-dependent relaxation has been demonstrated to be involved in the regulation of vascular tone and extracellular Ca2+ was found to play a prominent role in this process. Since the dependency on extracellular Ca2+ appeared to differ considerably within the arterial tree, possibly as a consequence of vessel-related endothelium-dependent mechanisms, we investigated the effects of different compounds affecting Ca2+ entry (nifedipine, CoCl2) on angiotensin II-induced contractions of rat aortic rings with and without endothelium as well as the responses in a Ca(2+)-"free" solution. For this purpose, rat aortic rings were either undone from their endothelial layer by gentle mechanical rubbing or care was taken to keep the intima intact in case rings where endothelium were required. The presence of an intact endothelium was confirmed by acetylcholine-induced relaxation. A stronger responsiveness towards angiotensin I, both after a complete concentration-response curve and after a single maximal concentration of angiotensin II was observed in arterial segments without endothelium. The maximal contraction to a single concentration of angiotensin II (0.1 microM) in the rings without endothelium amounted to 75.8 +/- 3.8% of the preceding response to a supramaximal concentration of noradrenaline (= Emax). In rings without the endothelial layer, the contraction was 34.8 +/- 3.7% of Emax. This indicates an endothelium-induced relaxation in aortic rings with endothelium. After incubation with the Ca2+ entry blocker nifedipine (1 microM) both rings with and without endothelium were inhibited to the same extent, contractions amounted to 30.7 +/- 1.8% and 19.6 +/- 1.3% of Emax, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes in Ca2+ channel antagonist receptors in rabbit lower urinary tract

(+)-[3H]PN 200-110 and [3H]nitrendipine binding sites were studied in 1 day, 6 week, 6 month and 4.5-5 year rabbit urethra, bladder base and bladder dome. In all age groups, the density of these Ca2+ channel antagonist binding sites was significantly greater in the urethra than in the bladder base or bladder dome, except that the density of [3H]nitrendipine binding sites in the 1 day rabbit was similar in all three regions. In the urethra the maximum number of binding sites for (+)-[3H]PN 200-110 was twice that for [3H]nitrendipine. In the bladder base and bladder dome both Ca2+ channel antagonists labelled a similar number of binding sites. For both ligands, the Bmax values for the Ca2+ channel antagonists in the urethra increased from 1 day to 6 weeks and then decreased. In the bladder base and dome, however, the receptor density decreased gradually or was unchanged with aging. The pharmacological profiles of these binding sites in 1 day and 6 month urethra and bladder dome showed K1 values with the following rank order: nitrendipine < niguldipine < BAY K 8644. These data demonstrate the presence of regional- and age-dependent changes in the density of Ca2+ channel antagonist binding sites in the lower urinary tract of the rabbit.

Cerebral vasoconstrictor mediators

Endogenous cerebral vasoconstrictor mediators regulate vascular resistance and blood flow in the brain as a whole and in various regions and participate in the pathogenesis of cerebral circulatory disturbances. Vasoconstrictors are effective in the treatment of diseases associated with cerebral vasodilatation. There are variations in the response of cerebral arteries from primate and subprimate mammals; therefore, information as to similarities and differences in their response is quite important in evaluating the physiological role, involvement in pathogenesis and therapeutic usefulness of the mediators in healthy men and patients. In this review we described characteristics of the action of vasoconstrictors (amines, peptides, prostanoids, and others) on isolated cerebral arteries from mammals, including humans and monkeys.

Relaxation effects of diltiazem, verapamil, and tolazoline on isolated cat ophthalmociliary artery

This study investigates the ability of an alpha 1-adrenergic blocker, tolazoline, two Ca2+ entry channel blockers, verapamil and diltiazem, acetylcholine and Ca(2+)-free solutions to relax alpha 1-adrenergically activated cat ophthalmociliary artery ring segments. The form of a contraction elicited with adrenaline, noradrenaline or phenylephrine was shown to depend on the passive tension set. As passive tension was increased, which is equivalent to increasing blood pressure in vivo, the alpha 1-adrenergic elicited contraction became more sustained. Incubation with the Ca2+ channel blockers diltiazem and verapamil was ineffective in reducing the phasic response to the alpha 1-adrenergic agonists whereas addition of these blockers in increasing concentrations during a sustained adrenergic contraction caused a graded and significant relaxation. This implies that therapeutic use of Ca2+ channel blockers may act directly on the ophthalmociliary artery to inhibit the autonomic control of sustained tone in this vessel. Removal of exogenous Ca2+ from the bathing medium initially reduced the phasic component and eliminated the tonic component of the alpha 1-adrenergic contraction. Repeated exposure of the ring segment to Ca(2+)-free medium further reduced the phasic component elicited with alpha 1-adrenergic contraction. Pre-incubation with the alpha 1-adrenergic blocker Tolazoline reduced the alpha 1-adrenergic antagonist phasic response only at high concentrations (10(-4) M), whereas Tolazoline applied during the tonic phase was an effective relaxant at lower concentrations. Acetylcholine added to the bath during an alpha 1-adrenergic contraction induced relaxation at low concentrations and contraction for higher concentrations. The relaxation was eliminated by removal of the endothelial cell layer. These data support the notion that the phasic component of an alpha 1-adrenergic contraction is mainly dependent on intracellular Ca2+ stores whereas the tonic component relies almost exclusively on extracellular Ca2+. It is suggested that Ca2+ channel blockers and alpha 1-adrenergic antagonists may potentially play a role in relaxing the ophthalmociliary artery to improve the ocular circulation in vascular diseases in which there is considerable vascular tone present.

Phenylephrine-induced translocation of protein kinase C and shortening of two types of vascular cells of the ferret

1. The relationship between phenylephrine-induced smooth muscle contraction and the subcellular distribution of protein kinase C (PKC) was investigated. 2. Cell shortening induced by phenylephrine (10(-5) M) was measured in single vascular cells freshly isolated from ferret portal vein and aorta. 3. At various time points during phenylephrine activation, single cells were fixed with paraformaldehyde and the distribution of PKC was imaged in cells labelled with the fluorescent PKC probe 12-(1,3,5,7-tetramethylBODIPY-2-propionyl)phorbol-13-acetate. 4. The PKC probe located to a perinuclear region, the cytosol and surface membrane. The amplitude and time course of the phenylephrine-induced changes in the surface membrane/cytosol fluorescence ratio were measured and compared with the amplitude and time course of phenylephrine-induced cell shortening. 5. In portal vein cells incubated in 1 mM-external Ca2+, phenylephrine caused significant shortening and time-dependent translocation of the PKC probe to the surface membrane, but cell shortening preceded PKC translocation. In Ca2+free solution both cell shortening and translocation of the probe were completely inhibited. 6. Verapamil (3 x 10(-7) M) partially, but significantly, inhibited the magnitude of cell shortening and delayed the onset and time to peak shortening. Translocation of PKC in verapamil preceded or coincided with cell shortening. 7. In aorta cells incubated in 1 mM-extracellular Ca2+, phenylephrine induced significant shortening and time-dependent translocation of the PKC probe. Cell shortening preceded PKC translocation. In Ca(2+)-free solution, shortening was only partially, but significantly, inhibited and PKC translocation preceded the fraction of the shortening response that remained. 8. These data are consistent with a role for PKC in the maintenance of the phenylephrine-induced contraction in both portal vein and aorta. The data also suggest that phenylephrine-induced contraction may involve activation of a Ca(2+)-dependent PKC isoform in ferret portal vein but a Ca(2+)-independent isoform in ferret aorta.

The effect of chronic and acute administration of deuterium oxide (D2O) on vascular smooth muscle contraction in spontaneously hypertensive and Wistar-Kyoto rats

1. Oral administration of 25% D2O for 12 days reduced blood pressure of spontaneously hypertensive rats (SHR) to the level of Wistar-Kyoto (WKY) controls. 2. However, the chronic D2O treatment appeared to have little effect on the phenylephrine and potassium chloride induced dose-response curves of SHR and WKY rats, producing a decreased maximal contraction of the potassium chloride dose-response curve of SHR only. 3. Further acute studies revealed that desensitization results from chronic exposure to D2O such that 60% D2O produces a significant depression of contraction only in aortic rings obtained from SHR and WKY which had not been chronically treated with 25% D2O.

The binding and functional properties of voltage dependent calcium channel receptors in pediatric normal and myelodysplastic bladders

The present study was designed to compare the binding and functional properties of calcium channel receptors in normal and myelodysplastic bladders. Normal bladders were obtained from children with vesicoureteral reflux undergoing ureteral reimplantation. Myelodysplastic bladder specimens were obtained from patients undergoing bladder augmentation. The functional studies included agonist (calcium chloride) dose response experiments and the determination of apparent antagonist dissociation constants for various calcium channel antagonists. The receptor binding studies were performed using the ligand (+)-3H-PN200-110 (specific activity 86.6 Ci./mmol.). The mean maximal response of myelodysplastic bladders to calcium ions was 31% less than normal bladders (p greater than 0.05). The mean EC50 for calcium mediated isometric tension and the mean -log antagonist dissociation constant values of nifedipine, diltiazem and verapamil were similar in normal and myelodysplastic bladders. The radioligand receptor binding studies demonstrated that the equilibrium dissociation constant of (+)-3H-PN200-110 in myelodysplastic bladders was 4-fold greater than in normal bladders. The density of dihydropyridine binding sites in myelodysplastic and normal bladders was similar. Our study demonstrated that the pathophysiology of the poorly compliant hyperreflexic bladder is not related to up regulation of dihydropyridine calcium channel receptors or alterations in the response of detrusor muscle to calcium ions. The relative abundance of calcium channel receptors in the normal and myelodysplastic bladders, and the regulation of detrusor contraction by calcium ions suggest that calcium channel receptors have a meaningful role in detrusor function.

Platelet-free calcium and vascular calcium uptake in ethanol-induced hypertensive rats

This study examined the effect of moderate ethanol intake on systolic blood pressure, platelet cytosolic free calcium, aortic calcium, and rubidium-86 uptake in Wistar-Kyoto rats. Twelve Wistar-Kyoto rats, aged 6 weeks, were given 5% ethanol in drinking water the first week followed by 10% ethanol in drinking water for the next 6 weeks. Twelve control animals were given regular tap water. Systolic blood pressure in the ethanol-treated rats was significantly higher (p less than 0.05) than that in controls after 1 week and remained higher throughout the study. At 13 weeks of age, platelet cytosolic free calcium and calcium uptake by aortas were significantly higher (p less than 0.001) in ethanol-treated animals as compared with those in controls. Ethanol intake did not affect aortic ouabain-sensitive 86Rb uptake. The in vitro effect of ethanol on calcium-45 and 86Rb uptake was also investigated in aortas of untreated Wistar-Kyoto rats at 13 weeks of age. In vitro ethanol (2.5-20 mmols/l) did not significantly affect 45Ca and 86Rb uptake in rat aortas. The increases in systolic blood pressure, platelet cytosolic free calcium, and vascular calcium uptake suggest that increases in cytosolic free calcium and calcium uptake mechanisms are associated with ethanol-induced hypertension.

Angiotensin II-induced increase in slowly exchanging 45Ca2+ in relation to contractile responses of rat and guinea-pig aorta

To gain more information about sources of activator Ca2+ involved in the contraction of rat and guinea-pig aorta evoked by angiotensin II and their sensitivity to Ca2+ entry blockers, measurement of slowly exchanging 45Ca2+ was established. A more physiological procedure was used, replacing La(3+)- and EGTA-containing solutions by a normal Ca(2+)-containing buffer. It was demonstrated that the angiotensin II-induced increase in slowly exchanging 45Ca2+ in rat aorta was incompletely (by approximately 60%-70%) inhibited by the organic Ca2+ entry blockers nifedipine, verapamil and diltiazem and by other Ca2+ entry blocking compounds like CoCl2 and chlorpromazine. 8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8) was able to inhibit the angiotensin II-induced increase in 45Ca2+ content completely, but this may be an intracellular storage effects. By contrast, the organic Ca2+ entry blockers completely inhibited that part of the angiotensin II-induced contraction of rat aorta which was dependent upon extracellular Ca2+. In guinea-pig aorta, the increase in 45Ca2+ content elicited by angiotensin II could be completely suppressed by all compounds under study. The results of these experiments correlated well with data from the functional experiments in guinea-pig aorta. In both preparations the release of Ca2+ from a rapidly as well as a slowly exchanging intracellular pool appears to contribute to the contractile response elicited by angiotensin II.

Heparin lowers blood pressure and vascular calcium uptake in hypertensive rats

Increased calcium uptake by vascular tissue, leading to elevated cytosolic calcium, has been implicated in the pathophysiology of hypertension. Heparin treatment of hypertensive rats has been known to lower blood pressure but its mechanism is not known. This study examined the effect of chronic heparin treatment on systolic blood pressure, aortic calcium and 87Rubidium (86Rb) uptake of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. Starting at 12 weeks of age SHR and WKY rats were given either sodium heparin 300 units s.c. or equal amounts of saline once a day for a period of 6 weeks. At 18 weeks, systolic blood pressure, uptakes of calcium and 86Rb by aortae were significantly higher (p less than 0.01) in saline-treated SHR compared with heparin-treated SHR and WKY. Heparin treatment lowered the elevated calcium and 86Rb Uptake and blood pressure in SHR but had no effect on WKY. The parallel increase in systolic blood pressure and vascular calcium uptake suggests that increased calcium uptake mechanisms are associated with hypertension in SHR. Heparin appears to lower elevated blood pressure in SHR by lowering elevated vascular calcium uptake.

Comparison of the effects of milrinone and OPC 3911 with those of isoprenaline, forskolin and dibutyryl-cAMP in rat aorta

1. OPC 3911 and milrinone, inhibitors of a cyclic nucleotide phosphodiesterase, and isoprenaline were more potent against contractions induced by phenylephrine (1 microM) than by K+ (60 mM). A similar tendency was found for dibutyryl-cAMP (db-cAMP) and forskolin, while the opposite was evident for nifedipine and diltiazem. 2. Contractions induced by Bay K 8644 (0.1 microM), in the presence of K+ (12 mM), were abolished by OPC 3911 (10 microM), milrinone (10 microM), db-cAMP (100 microM) and forskolin (1 microM). These agents had little effect on contractions induced by Bay K 8644 in the presence of K+ (20 mM), whereas diltiazem (10 microM) caused complete inhibition. 3. In nominally Ca(2+)-free medium, OPC 3911 (10 microM), milrinone (10 microM), db-cAMP (100 microM) and forskolin (1 microM) reduced phenylephrine-induced contractions. Db-cAMP and forskolin also attenuated contractions elicited by caffeine (20 mM). 4. Pretreatment by ryanodine (10 microM) reduced the effects of OPC 3911 (10 microM), milrinone (10 microM) and forskolin (1 microM) on phenylephrine-induced contractions.

Characterization of 1,4, dihydropyridine calcium channel binding sites in the human prostate

The binding and functional properties of calcium channel receptors have not been previously characterized in the normal or hyperplastic prostate. Dihydropyridine (DHP) binding sites have been characterized in other tissues using the ligands 3H-nitrendipene and (+)3H-PN200-110. Saturation experiments were performed on homogenates obtained from five human prostate adenomas using these ligands. The binding of 3H-nitrendipine and (+)3H-PN200-110 in the prostate was saturable and of high affinity. The mean Kd of 3H-nitrendipine and (+)3H-PN200-110 was 0.92 +/- 0.11 nM and 0.14 +/- 0.02 nM, respectively. The mean Bmax of 3H-nitrendipine and (+)3H-PN200-110 was 0.57 +/- 0.06 and 0.19 +/- 0.02 fmol/mg. wet wt., respectively. The percent specific binding of 3H-nitrendipene and (+)3H-PN200-110 was 18 +/- 1% and 38 +/- 4%, respectively. The pharmacology of (+)3H-PN200-110 binding sites was further characterized using competition displacement experiments. The IC50 corrected values for Bay K 8644, nifedipine, verapamil, and diltiazem in the human prostate and other tissues are of the same order of magnitude. The prostate contains an abundance of high affinity DHP binding sites. The physiologic significance of the DHP binding sites in the prostate requires further investigation.

Calcium channels, potassium channels, and voltage dependence of arterial smooth muscle tone

Resistance arteries exist in a maintained contracted state from which they can dilate or constrict depending on need. In many cases, these arteries constrict to membrane depolarization and dilate to membrane hyperpolarization and Ca-channel blockers. We discuss recent information on the regulation of arterial smooth muscle voltage-dependent Ca channels by membrane potential and vasoconstrictors and on the regulation of membrane potential and K channels by vasodilators. We show that voltage-dependent Ca channels in the steady state can be open and very sensitive to membrane potential changes in a range that occurs in resistance arteries with tone. Many synthetic and endogenous vasodilators act, at least in part, through membrane hyperpolarization caused by opening K channels. We discuss evidence that these vasodilators act on a common target, the ATP-sensitive K (KATP) channel that is inhibited by sulfonylurea drugs. We propose the following hypotheses that presently explain these findings: 1) arterial smooth muscle tone is regulated by membrane potential primarily through the voltage dependence of Ca channels; 2) many vasoconstrictors act, in part, by opening voltage-dependent Ca channels through membrane depolarization and activation by second messengers; and 3) many vasodilators work, in part, through membrane hyperpolarization caused by KATP channel activation.

Calcium antagonist-induced relaxation of the prostaglandin-F2 alpha response of isolated calf retinal resistance arteries

The relaxing effect of nitrendipine and D600 on isolated ring segments of calf retinal resistance arteries (i.d. ca. 200 microns) contracted with prostaglandin-F2 alpha (PGF2 alpha) or high potassium solution was studied. The vessel response to both PGF2 alpha and potassium stimulation was dependent on extracellular calcium. Removal of extracellular calcium reduced the vessel response to PGF2 alpha and potassium by 85% and 98%, respectively (P less than 0.01). Both calcium antagonists induced a concentration dependent relaxation of PGF2 alpha pre-contracted vessels with = -log[IC50(M)]-values of 8.01 and 7.13 for nitrendipine and D600 (P less than 0.05), respectively. The calcium antagonists were equieffective in relaxing the vessels amounting to 33% for nitrendipine at 10(-6) M and 37% for D600 at 10(-5) M. Further analysis of the data revealed that the nitrendipine induced relaxation of the PGF2 alpha response was linearly correlated with the internal lumen diameter of the vessels. No correlation was found for the D600 induced relaxation. At the highest concentrations of nitrendipine, 10(-6) M, and D600, 10(-5) M, the potassium induced response was reduced by 94 and 75%, respectively. The results show that calcium antagonists only partially relax retinal resistance arteries contracted with PGF2 alpha although the vessels are dependent on extracellular calcium for active force generation. Calcium must therefore be activated by PGF2 alpha through other pathways than those blocked by calcium antagonists.

Contractile effects of substance P and other tachykinins on the mouse isolated distal colon

1. Substance P (SP), physalaemin, eledoisin and kassinin induced concentration-related contractions of the longitudinal muscle of the mouse distal colon. The responses were not antagonized by atropine (1.5 x 10(-7) M), mepyramine (2.5 x 10(-7) M), methysergide (5 x 10(-7) M), timolol (10(-6) M), phentolamine (10(-6) M) or naloxone (4 x 10(-7) M). They were enhanced by tetrodotoxin (TTX, 1.5 x 10(-7) M). These observations indicate that the contractile responses to the tachykinins result from a direct activation of smooth muscle cells. 2. The contractile activity provoked by SP and physalaemin was inhibited by nifedipine (a Ca2+-entry blocker) and was abolished in Ca2+-free EGTA solution. Such data suggest that the myogenic effects of SP and physalaemin are mainly dependent on their ability to promote Ca2+ influx. 3. Eledoisin and kassinin evoked a contractile response in the absence of external Ca2+ and their myogenic activity was, to some extent, resistant to the inhibitory effect of nifedipine. This may indicate that an additional process, probably the release of an intracellularly bound Ca2+ store, participates in the mechanism by which eledoisin and kassinin contract the mouse distal colon. 4. After desensitization of the mouse distal colon to SP, the contractile activity provoked by SP or physalaemin was totally abolished whilst the responses evoked by eledoisin and kassinin were barely affected. These observations and other experimental findings indirectly support the assumption that the mouse distal colon could possess different tachykinin-binding sites.