Effects of Ca2+ channel blockers on Ca2+ loading induced by metabolic inhibition and hyperkalemia in cardiomyocytes

Berberine Mediated Positive Inotropic Effects on Rat Hearts via a Ca2+-Dependent Mechanism

Previous studies showed that berberine, an alkaloid from Coptis Chinensis Franch, might exert a positive inotropic effect on the heart. However, the underlying mechanisms were unclear. Here, we reported that berberine at 10–20 µM increased the left ventricular (LV) developed pressure and the maximal rate of the pressure rising, and it increased the maximal rate of the pressure descending at 20 µM in Langendorff-perfused isolated rat hearts. These effects diminished with the concentration of berberine increasing to 50 µM. In the concentration range of 50–300 µM, berberine increased the isometric tension of isolated left ventricular muscle (LVM) strips with or without electrical stimulations, and it (30–300 µM) also increased the intracellular Ca²⁺ level in the isolated LV myocytes. The removal of extracellular Ca²⁺ hindered the berberine-induced increases in the tension of LVM strips and the intracellular Ca²⁺ level of LV myocytes. These suggested that berberine might exert its positive inotropic effects via enhancing Ca²⁺ influx. The blockade of L-type Ca²⁺ channels (LTCCs) with nifedipine significantly attenuated 300 μM berberine-induced tension increase in LVM strips but not the increase in the intracellular Ca²⁺ level. Berberine (300 μM) further increased the LVM tension following the treatment with the LTCC opener FPL-64716 (10 μM), indicating an LTCC-independent effect of berberine. Lowering extracellular Na⁺ attenuated the berberine-induced increases in both the tension of LVM strips and the intracellular Ca²⁺ level of LV myocytes. In conclusion, berberine might exert a positive inotropic effect on the isolated rat heart by enhancing the Ca²⁺ influx in LV myocytes; these were extracellular Na⁺-dependent.

Metabolic inhibition reduces cardiac L-type Ca2+ channel current due to acidification caused by ATP hydrolysis

Metabolic stress evoked by myocardial ischemia leads to impairment of cardiac excitation and contractility. We studied the mechanisms by which metabolic inhibition affects the activity of L-type Ca²⁺ channels (LTCCs) in frog ventricular myocytes. Metabolic inhibition induced by the protonophore FCCP (as well as by 2,4- dinitrophenol, sodium azide or antimycin A) resulted in a dose-dependent reduction of LTCC current (I_Ca,L) which was more pronounced during β-adrenergic stimulation with isoprenaline. I_Ca,L was still reduced by metabolic inhibition even in the presence of 3 mM intracellular ATP, or when the cell was dialysed with cAMP or ATP-γ-S to induce irreversible thiophosphorylation of LTCCs, indicating that reduction in I_Ca,L is not due to ATP depletion and/or reduced phosphorylation of the channels. However, the effect of metabolic inhibition on I_Ca,L was strongly attenuated when the mitochondrial F₁F₀-ATP-synthase was blocked by oligomycin or when the cells were dialysed with the non-hydrolysable ATP analogue AMP-PCP. Moreover, increasing the intracellular pH buffering capacity or intracellular dialysis of the myocytes with an alkaline solution strongly attenuated the inhibitory effect of FCCP on I_Ca,L. Thus, our data demonstrate that metabolic inhibition leads to excessive ATP hydrolysis by the mitochondrial F₁F₀-ATP-synthase operating in the reverse mode and this results in intracellular acidosis causing the suppression of I_Ca,L. Limiting ATP break-down by F₁F₀-ATP-synthase and the consecutive development of intracellular acidosis might thus represent a potential therapeutic approach for maintaining a normal cardiac function during ischemia.

The origin of calcium overload in rat cardiac myocytes following metabolic inhibition with 2,4-dinitrophenol

We have investigated the characteristics of the rise in cytoplasmic calcium that occurs when rat isolated cardiac ventricular myocytes are exposed to 2,4-dinitrophenol using conventional and confocal fluorescence microscopy and patch clamp. 2,4-dinitrophenol (200 microM) caused cytoplasmic calcium to increase in two phases: (1) an initial rise in fluo-3 fluorescence of 36+/-2% that was maintained until rigor contraction; (2) a further progressive rise so that fluo-3 fluorescence had increased by 177+/-12% 535 s after 2,4-dinitrophenol addition. Both phases were unaffected by removal of external Ca(2+). 2,4-dinitrophenol caused mitochondrial depolarization, measured using tetramethyl rhodamine ethyl ester fluorescence. Mitochondrial depolarization was associated with a decrease in intra-mitochondrial calcium measured with rhod-2, and experiments on myocytes loaded with both fluo-3 and rhod-2 showed that fluo-3 fluorescence increased as rhod-2 fluorescence fell. The correlation of the onset of the second phase of the increase in cytoplasmic calcium with rigor suggested that this phase was consequent on ATP depletion. DNP also caused activation of an ATP-sensitive potassium current. Depletion of sarcoplasmic reticulum calcium stores by pretreatment with ryanodine, thapsigargin and caffeine prior to the addition of 2,4-dinitrophenol did not affect the initial increase in cytoplasmic calcium, but abolished phase 2. Our results suggest that the initial rise in cytoplasmic calcium seen on application of 2,4-dinitrophenol results from release of mitochondrial calcium because of mitochondrial depolarization, while the second phase is caused by progressive release of calcium from the sarcoplasmic reticulum following depletion of intracellular ATP.

Lead–cadmium interaction effect on the responsiveness of rat mesenteric vessels to norepinephrine and angiotensin II

The comparison of the reactivity to norepinephrine (NE) and angiotensin II (A II) of isolated mesenteric blood vessels obtained from rats simultaneously poisoned with lead and cadmium to those responses of rats treated singly with lead or cadmium was performed. Male Buffalo rats aged 6-8 weeks were administered intragastrically with lead (35 mg Pb/kg body wt.) and/or cadmium (5 mg Cd/body wt.), once a week for a period of 7 weeks. Control rats were given equimolar amounts of sodium acetate and/or sodium chloride. Changes in mesenteric vascular resistance due to NE and A II injections were measured ex vivo as an increase in perfusion pressure in vessels prepared by McGregor's method. The dose-response curve for NE (0.01-5.0 microg) determined for vessels of rats poisoned simultaneously with lead and cadmium was shifted to the left in comparison to controls (not poisoned rats), similarly to these determined for rats poisoned with lead or cadmium. ED(50) NE pointed out in the control group (0.83+/-0.5 microg) was significantly greater than in metal treated groups (0.44+/-0.09; 0.45+/-0.26 and 0.5+/-0.11 microg in lead, cadmium, lead and cadmium-treated rats, respectively). This study indicated a tachyphylaxis in responses of isolated mesenteric vessels to A II injected in increasing doses, and the weaker, in comparison to controls, response of vessels of rats poisoned with lead and/or cadmium to A II at a dose of 0.4 microg. The decreasing response to A II could result from changes in calcium ions transport through L-type channels in vascular smooth muscle cells, because verapamil (2.0 microM) inhibited the A II-induced vasoconstriction more weakly in rats poisoned with metals than in controls. Inhibitor of prostaglandins synthesis, ketoprofen (200 microg/ml per min.) attenuated the pressor effect of NE in blood vessels obtained from all rats, but this effect was less potent in arteries of cadmium poisoned rats. Ketoprofen also inhibited the vasoconstrictory action of A II in all groups, but this effect was lower in vessels of rats poisoned simultaneously with lead and cadmium. We suggest that the release of vasoactive prostaglandins as a consequence of endothelial angiotensin receptor stimulation changes more under the influence of metals administered to rats simultaneously than under the influence or lead or cadmium administered singly. Treatment with a nitric oxide synthase inhibitor (L-NOARG; 22 microg/ml per min.) potentiated a NE-induced pressor response in all groups. However, the increase in perfusion pressure was greater in rats poisoned with cadmium in comparison to controls. L-NOARG potentiated the A II induced vasoconstriction only in cadmium poisoned rats, also indicating a greater influence of nitric oxide in cadmium treated rat vasculature. Two-way ANOVA showed the existence of lead-cadmium interactions effects on the reactivity of rat isolated mesenteric vessels to NE, A II and papaverine.

Protection of cardiomyocytes by pinacidil during metabolic inhibition and hyperkalemia

The objective of this study is to understand the mechanism underlying the cardioprotective effects of pinacidil, an ATP-sensitive K+ channel (K(ATP)) opener. We examined the effects of 10 microM pinacidil in cultured chicken cardiomyocytes. Pinacidil caused a concentration-dependent delay in metabolic inhibition-induced increase in intracellular calcium concentration ([Ca2+]i) and creatine phosphokinase release, and this action was antagonized by glyburide, a K(ATP) blocker. Neither verapamil, an L-type Ca2+ channel blocker, nor bepridil, a Na+-Ca2+ exchange inhibitor, affected the time course of increase in [Ca2+]i induced by metabolic inhibition. Pinacidil did not have an effect on the amplitude of K+-induced increase in [Ca2+]i, but accelerated the rate of decline following peak stimulation. In contrast, glyburide reduced the amplitude of K+-induced increase in [Ca2+]i and prolonged the rate of decline. These results provide direct evidence that pinacidil protects cardiomyocytes from metabolic inhibition-induced injury by cyanide (CN) through a delay in the onset of increase in [Ca2+]i, rather than by inhibition of the L-type Ca2+-channels or by alteration of Na+-Ca2+ exchange.