Free intracellular calcium in essential hypertension. Effects of nifedipine and captopril

The acute effects of nifedipine (20 mg sublingually) and of captopril (12.5 mg orally) on blood pressure and on platelet intracellular free calcium were investigated in 11 and 12 patients, respectively, with essential hypertension. Platelet calcium was measured by the Quin 2 method. Application of both drugs resulted in a significant fall in blood pressure within 60 min. Platelet calcium, however, was lowered by nifedipine only. There was no correlation between blood pressure reduction and changes in platelet calcium. Platelet intracellular free calcium concentration was 99 +/- 3 nmol in 30 normotensive subjects and 109 +/- 5 nmol (+/- s.e.m.) in 26 patients with essential hypertension (P < 0.05). There was a wide overlap between the values of normotensives and hypertensives. Thus there is no positive evidence of an increased platelet intracellular free calcium concentration in a large proportion of patients with essential hypertension.

Effect of nifedipine and verapamil on alpha-receptor-activation in patients with essential hypertension

The question of whether the hypotensive effect of calcium entry blockers involves an interaction with alpha-adrenergic receptors was examined. The effect of nifedipine subl. (20 mg, n = 9) and of verapamil p.o. (160 mg, n = 9) on the pressor effect of the unselective alpha-adrenergic agonist noradrenaline, as well as on 3H-yohimbine binding to platelet alpha 2-adrenoceptors was studied in patients with essential hypertension. In addition, the effect of nifedipine on reactivity to the selective alpha 1-adrenergic agonist phenylephrine was investigated (n = 9). Nifedipine caused a significant reduction of reactivity to noradrenaline (P less than 0.01), along with a significant decrease in binding sites (P less than 0.01). Affinity to the alpha 2-receptors was unchanged. Verapamil, although equally effective in lowering blood pressure, had no effect on the pressor response or binding sites. The pressor effect of the alpha 1-agonist phenylephrine was reduced (P less than 0.01) by nifedipine. Nifedipine may therefore affect both alpha 1- and alpha 2-adrenoceptors in patients with essential hypertension. Since verapamil did not affect the pressor response to noradrenaline or yohimbine-binding, the interaction with alpha 2-adrenoceptors does not appear to be a general prerequisite for the hypotensive action of calcium entry blockers.

Characterization of Mg-ATP-dependent Ca2+ transport in cat pancreatic microsomes

Mg-ATP-dependent 45Ca2+ uptake and Ca2+-ATPase activity have been examined in isolated microsomes obtained by differential centrifugation and in purified subcellular fractions obtained by Ficoll-sucrose density centrifugation in the presence of mitochondrial inhibitors. Mg-ATP-dependent 45Ca2+ uptake increased with increasing EGTA-buffered free [Ca2+], reaching a maximum of 2 nmol 45Ca2+ X 15 min-1 X mg prot-1 at 2 mumol/1 [Ca2+] in the incubation medium. Half-maximal 45Ca2+ uptake was at approximately 0.2 mumol/1 [Ca2+]. Maximal Ca2+ -Mg2+ -ATPase activity was 130 nmol X 15 min-1 X mg prot-1 at 2 mumol/l [Ca2+], with an apparent Km of approximately 0.3 mumol/l [Ca2+]. The Ca2+ ionophore A23187 (10(-6) mol/l), the mercurial compounds mersalyl (10(-5) mol/l) and CH3ClHg (10(-3) mol/l), as well as La3+ (10(-4) mol/l), vanadate (10(-4) mol/l), and saponin (50 micrograms/mg prot), abolished Mg-ATP-promoted 45Ca2+ uptake. In the absence of Mg2+, ATP did not provoke 45Ca2+ uptake. Using the purified smooth membrane fraction (F1) from the Ficoll-sucrose density gradient (enrichment of Na+-K+-ATPase specific activity by ninefold and of NADH-cytochrome c reductase by threefold as compared with total tissue homogenate), Mg-ATP-dependent 45Ca2+ uptake correlated better with Na+-K+-ATPase (r = 0.97) than with the smooth endoplasmic marker NADH-cytochrome c reductase (r = 0.52). No correlation was found with RNA, the marker for rough endoplasmic reticulum. We conclude that pancreatic plasma membranes contain a Ca2+-Mg2+-ATPase that represents the Ca2+ extrusion system from acinar cells. It is also possible that vesicular membrane structures associated with the plasma membrane, or endocytotic plasma membrane vesicles, take up Ca2+ and represent an intracellular Ca2+ pool.

Calcium uptake into acini from rat pancreas: evidence for intracellular ATP-dependent calcium sequestration

Intracellular ATP-dependent Ca2+-sequestration mechanisms were studied in isolated dispersed rat pancreatic acini following treatment with saponin or digitonin to disrupt their plasma membranes. In the presence of 45Ca2+ concentrations less than 10(-6) mol/liter, addition of 5 mmol/liter ATP caused a rapid increase in 45Ca2+ uptake exceeding the control by fivefold. ADP mimicked the ATP effect by 50 to 60%, whereas other nucleotides such as AMP-PNP, AMP-PCP, CTP, UTP, ITP, GTP, cAMP and cGMP did not. Maximal ATP-promoted Ca2+ uptake was obtained at 10(-5) mol/liter Ca2+. Inhibition of Ca2+ uptake by mitochondrial inhibitors was dependent on the Ca2+ concentration, indicating the presence of different Ca2+ storage systems. Whereas the apparent half-saturation constant found for mitochondrial Ca2+ uptake was approximately 4.5 X 10(-7) mol/liter, in the presence of antimycin and oligomycin (nonmitochondrial uptake) it was approximately 1.4 X 10(-8) mol/liter. In the absence of Mg2+ both ATP- and ADP-promoted Ca2+ uptake was nearly abolished. The Ca2+ ionophore and mersalyl blocked Ca2+ uptake, Electron microscopy showed electron-dense precipitates in the rough endoplasmic reticulum of saponin-treated cells in the presence of Ca2+, oxalate and ATP, which were absent in intact cells and in saponin-cells without ATP or pretreated with A23187. The data suggest the presence of mitochondrial and nonmitochondrial ATP-dependent C2+ storage systems in pancreatic acini. The latter is likely to be located in the rough endoplasmic reticulum.

Analysis of Ca2+ fluxes and Ca2+ pools in pancreatic acini

45Ca2+ movements have been analysed in dispersed acini prepared from rat pancreas in a quasi-steady state for 45Ca2+. Carbamyl choline (carbachol; Cch) caused a quick 45Ca2+ release that was followed by a slower 45Ca2+ 'reuptake'. Subsequent addition of atropine resulted in a further transient increase in cellular 45Ca2+. The data suggest the presence of a Cch-sensitive 'trigger' pool, which could be refilled by the antagonist, and one or more intracellular 'storage' pools. Intracellular Ca2+ sequestration was studied in isolated acini pretreated with saponin to disrupt their plasma membranes. In the presence of 45Ca2+ (1 microM), addition of ATP at 5 mM caused a rapid increase in 45Ca2+ uptake exceeding the control by fivefold. Maximal ATP-promoted Ca2+ uptake was obtained at 10 microM Ca2+ (half-maximal at 0.32 microM Ca2+). In the presence of mitochondrial inhibitors it was 0.1 microM (half-maximal at 0.014 microM). 45Ca2+ release could still be induced by Cch but the subsequent reuptake was missing. The latter was restored by ATP and atropine caused further 45Ca2+ uptake. Electron microscopy showed electron-dense precipitates in the rough endoplasmic reticulum of saponin-treated cells in the presence of Ca2+, oxalate and ATP which were absent in intact cells or cells pretreated with A23187. The data suggest the presence of a plasma membrane-bound Cch-sensitive 'trigger' Ca2+ pool and ATP-dependent Ca2+ storage systems in mitochondria and rough endoplasmic reticulum of pancreatic acini. It is assumed that Ca2+ is taken up into these pools after secretagogue-induced Ca2+ release.U