The specific inhibition of the cardiac electrogenic sodium/bicarbonate cotransporter leads to cardiac hypertrophy.

Two alkalinizing mechanisms coexist in cardiac myocytes to maintain intracellular pH: sodium/bicarbonate cotransporter (electroneutral isoform NBCn1 and electrogenic isoform NBCe1) and sodium/proton exchanger (NHE1). Dysfunction of these transporters has previously been reported to be responsible for the development of cardiovascular diseases. The aim of this study was to evaluate the contribution of the downregulation of the NBCe1 to the development of cardiac hypertrophy. To specifically reduce NBCe1 expression, we cloned shRNA into a cardiotropic adeno-associated vector (AAV9-shNBCe1). After 28 days of being injected with AAV9-shNBCe1, the expression and the activity of NBCe1 in the rat heart were reduced. Strikingly, downregulation of NBCe1 causes significant hypertrophic heart growth, lengthening of the action potential in isolated myocytes, an increase in the duration of the QT interval and an increase in the frequency of Ca2+ waves without any significant changes in Ca2+ transients. An increased compensatory expression of NBCn1 and NHE1 was also observed. We conclude that reduction of NBCe1 is sufficient to induce cardiac hypertrophy and modify the electrical features of the rat heart.

[The RAAS and SARS-CoV-2: A riddle to solve]. / El SRAA y el SARS-CoV-2: el acertijo a resolver.

The first case of COVID-19 was reported on 31 December 2019 in Wuhan, China. Ever since there has been unprecedented and growing interest in learning about all aspects of this new disease. Debate has been generated as to the association between antihypertensive therapy with renin-angiotensin-aldosterone system (RAAS) inhibitors and SARS-CoV-2 infection. While many questions as yet remain unanswered, the aim of this report is to inform health professionals about the current state of knowledge. Because this is an ever-evolving topic, the recommendation is that it be updated as new evidence becomes available. Below, we provide a review of pre-clinical and clinical studies that link coronavirus to the RAAS.

Mitochondrial reactive oxygen species (ROS) as signaling molecules of intracellular pathways triggered by the cardiac renin-angiotensin II-aldosterone system (RAAS).

Mitochondria represent major sources of basal reactive oxygen species (ROS) production of the cardiomyocyte. The role of ROS as signaling molecules that mediate different intracellular pathways has gained increasing interest among physiologists in the last years. In our lab, we have been studying the participation of mitochondrial ROS in the intracellular pathways triggered by the renin-angiotensin II-aldosterone system (RAAS) in the myocardium during the past few years. We have demonstrated that acute activation of cardiac RAAS induces mitochondrial ATP-dependent potassium channel (mitoKATP) opening with the consequent enhanced production of mitochondrial ROS. These oxidant molecules, in turn, activate membrane transporters, as sodium/hydrogen exchanger (NHE-1) and sodium/bicarbonate cotransporter (NBC) via the stimulation of the ROS-sensitive MAPK cascade. The stimulation of such effectors leads to an increase in cardiac contractility. In addition, it is feasible to suggest that a sustained enhanced production of mitochondrial ROS induced by chronic cardiac RAAS, and hence, chronic NHE-1 and NBC stimulation, would also result in the development of cardiac hypertrophy.

The positive inotropic effect of endothelin-1 is mediated by mitochondrial reactive oxygen species.

We have previously demonstrated the participation of reactive oxygen species (ROS) in the positive inotropic effect of a physiological concentration of Angiotensin II (Ang II, 1 nM). The objective of the present work was to evaluate the role and source of ROS generation in the positive inotropic effect produced by an equipotent concentration of endothelin-1 (ET-1, 0.4 nM). Isolated cat ventricular myocytes were used to measure sarcomere shortening with a video-camera, superoxide anion (()O(2)(-)) with chemiluminescence, and ROS production and intracellular pH (pH(i)) with epifluorescence. The ET-1-induced positive inotropic effect (40.4+/-3.1%, n=10, p<0.05) was associated to an increase in ROS production (105+/-29 fluorescence units above control, n=6, p<0.05). ET-1 also induced an increase in ()O(2)(-) production that was inhibited by the NADPH oxidase blocker, apocynin, and by the blockers of mitochondrial ATP-sensitive K(+) channels (mK(ATP)), glibenclamide and 5 hydroxydecanoic acid. The ET-1-induced positive inotropic effect was inhibited by apocynin (0.3 mM; 6.3+/-6.6%, n=13), glibenclamide (50 microM; 8.8+/-3.5%, n=6), 5 hydroxydecanoic acid (500 microM; 14.1+/-8.1, n=9), and by scavenging ROS with MPG (2 mM; 0.92+/-5.6%, n=8). ET-1 enhanced proton efflux (J(H)) carried by the Na(+)/H(+) exchanger (NHE) after an acid load, effect that was blocked by MPG. Consistently, the ET-induced positive inotropic effect was also inhibited by the NHE selective blocker HOE642 (5 microM; 9.37+/-6.07%, n=7). The data show that the effect of a concentration of ET-1 that induces an increase in contractility of about 40% is totally mediated by an intracellular pathway triggered by mitochondrial ROS formation and stimulation of the NHE.

Positive inotropic and negative lusitropic effect of angiotensin II: intracellular mechanisms and second messengers.

In the cat ventricle angiotensin II exerts a positive inotropic effect produced by an increase in intracellular calcium associated with a prolongation of relaxation. The signaling cascades involved in these effects as well as the subcellular mechanisms of the negative lusitropic effect are still not clearly defined. The present study was directed to investigate these issues in cat papillary muscles and isolated myocytes. The functional suppression of the sarcoplasmic reticulum (SR) with either 0.5 microm ryanodine or 0.5 microm ryanodine plus 1 microm thapsigargin or the preincubation of the myocytes with the specific inhibitor of the inositol 1,4,5-triphosphate (IP3) receptors [diphenylborinic acid, ethanolamine ester (2-APB), 5-50 microm] did not prevent the positive inotropic effect and the increment in Ca2+ transient produced by 1 microm angiotensin II. In contrast, protein kinase C (PKC) inhibitors, chelerythrine (20 microm) and calphostin C (1 microm) completely inhibited both, the angiotensin II-induced increase in L-type calcium current and positive inotropic effect. The prolongation of half relaxation time produced by 0.5 microm angiotensin II [207+/-15.4 msec (control) to 235+/-19.98 msec (angiotensin II), P<0.05] was completely blunted by PKC inhibition. This antirelaxant effect, which was independent of intracellular pH changes, was associated with a prolongation of the action potential duration and was preserved after either the inhibition of the SR and the SR Ca2+ ATPase (ryanodine plus thapsigargin) or of the reverse mode of the Na+/Ca2+ exchanger (KB-R7943, 5 microm). We conclude that in feline myocardium the positive inotropic and negative lusitropic effects of angiotensin II are both entirely mediated by PKC without any significant participation of the IP3 limb of the phosphatidylinositol/phospholipase C cascade. The results suggest that the antirelaxant effect of angiotensin II might be determined by the decrease in Ca2+ efflux through the Na+/Ca2+ exchanger produced by the angiotensin II-induced prolongation of the action potential duration.

Angiotensin II stimulates cardiac L-type Ca(2+) current by a Ca(2+)- and protein kinase C-dependent mechanism.

Angiotensin II (ANG II) evokes positive inotropic responses in various species. However, the effects of this peptide on L-type Ca(2+) currents (I(Ca)) are still controversial. We report in this study that the effects of ANG II on I(Ca) differ depending on the mode of patch-clamp technique used, standard whole cell (WC) or perforated patch (PP). No significant effects of ANG II (0.5 microM) were observed when WC in cells dialyzed with high EGTA was used. However, when the intracellular milieu was preserved using PP, ANG II induced a significant 77 +/- 6% increase in I(Ca) (-2.2 +/- 0.3 in control and -3.9 +/- 0.6 pA/pF in ANG II, n = 8, P < 0.05). When WC was used in cells dialyzed with low Ca(2+) buffer capacity (EGTA 0.1 mM), ANG II was able to induce an increase in I(Ca) (-3.5 +/- 0.3 in control vs. -4.8 +/- 0.4 pA/pF in ANG II, n = 13, P < 0.05). This increase was prevented when the cells were also dialyzed with the protein kinase C (PKC) inhibitor chelerythrine (50 microM) or calphostin C (1 microM). The above results allow us to conclude that strong intracellular Ca(2+) buffering prevents the physiological actions of ANG II on cardiac I(Ca), which are also dependent on activation of PKC.

Subcellular mechanisms of the positive inotropic effect of angiotensin II in cat myocardium.

1. Cat ventricular myocytes loaded with [Ca2+]i- and pHi-sensitive probes were used to examine the subcellular mechanism(s) of the Ang II-induced positive inotropic effect. Ang II (1 microM) produced parallel increases in contraction and Ca2+ transient amplitudes and a slowly developing intracellular alkalisation. Maximal increases in contraction amplitude and Ca2+ transient amplitude were 163 +/- 22 and 43 +/- 8 %, respectively, and occurred between 5 and 7 min after Ang II administration, whereas pHi increase (0.06 +/- 0.03 pH units) became significant only 15 min after the addition of Ang II. Furthermore, the inotropic effect of Ang II was preserved in the presence of Na+-H+ exchanger blockade. These results indicate that the positive inotropic effect of Ang II is independent of changes in pHi. 2. Similar increases in contractility produced by either elevating extracellular [Ca2+] or by Ang II application produced similar increases in peak systolic Ca2+ indicating that an increase in myofilament responsiveness to Ca2+ does not participate in the Ang II-induced positive inotropic effect. 3. Ang II significantly increased the L-type Ca2+ current, as assessed by using the perforated patch-clamp technique (peak current recorded at 0 mV: -1.88 +/- 0.16 pA pF-1 in control vs. -3.03 +/- 0.20 pA pF-1 after 6-8 min of administration of Ang II to the bath solution). 4. The positive inotropic effect of Ang II was not modified in the presence of either KB-R7943, a specific blocker of the Na+-Ca2+ exchanger, or ryanodine plus thapsigargin, used to block the sarcoplasmic reticulum function. 5. The above results allow us to conclude that in the cat ventricle the Ang II-induced positive inotropic effect is due to an increase in the intracellular Ca2+ transient, an enhancement of the L-type Ca2+ current being the dominant mechanism underlying this increase.

Chronic administration of nifedipine induces up-regulation of functional calcium channels in rat myocardium.

Previous studies from our laboratory demonstrated the up-regulation of cardiac dihydropyridine (DHP) receptors in rabbits chronically treated with nifedipine (NIFE). The goal of the present study was to further examine the functionality of this increased number of receptors by analysing different steps of excitation contraction coupling mechanism in adult rats chronically treated with NIFE (a single 10-mg oral dose/kg/day for 28 days). Ca2+ channel density was assessed by specific binding at the DHP receptors with [methyl-(3)H]PN 200-110 in rat ventricular membranes. Chronic NIFE treatment produced up-regulation of Ca2+ channels, being the maximal binding capacities 222+/-19 fmol/mg protein (n=14) and 310+/-21 fmol/mg protein (n=11) in untreated and treated animals, respectively (P<0.05). The functional consequences of this up-regulation of Ca2+ channels were determined in isolated ventricular myocytes by measuring L-type Ca2+ currents (I(Ca)) with the whole-cell configuration of patch-clamp technique and by intracellular Ca2+ (Ca2+(i)) transients estimated by the Indo-1/AM fluorescence ratio (410/482) simultaneously monitored with cell shortening. Peak I(Ca) density recorded at 0 mV was 32% greater in myocytes isolated from the treated group than in those obtained from the untreated group (-10.43+/-0.73 pA/pF (n=13) vs-7.10+/-0.59 pA/pF (n=12) P<0.05). Ca2+(i) transient amplitude and cell shortening, explored at 1 and 2 mM extracellular calcium ([Ca]0) were significantly higher in ventricular myocytes obtained fom NIFE-treated rats than in myocytes isolated from untreated animals. At 2 mM [Ca]0, the values of Ca2+(i) transient and shortening were 460+/-61 nM and 11+/-1 % of resting length (L(0)) in myocytes from treated rats (n=9) and 212+/-22 nM and 5.3+/-0.5% of L(0) in myocytes from control rats (n=6, P<0.05). The results demonstrate an up-regulation of functionally-active cardiac Ca2+ channels after NIFE treatment, and offer a possible explanation for a "withdrawal effect" at myocardial level after the suppression of the treatment with this drug.

Role of a Ca2+-activated K+ current in the maintenance of resting membrane potential of isolated, human, saphenous vein smooth muscle cells.

Calcium-activated potassium currents were studied in dissociated smooth muscle cells from human saphenous vein (HSV) using the patch-clamp technique in the whole-cell configuration. The average measured resting membrane potential (Vm) was -41+/-2 mV (n=39), when the cells were dialysed with an intracellular pipette solution (IPS) containing 0.1 mM ethyleneglycol-bis(beta-aminoethylether)-N,N,N', N'-tetraacetic acid (EGTA) (IPS-0.1 mM EGTA). When the EGTA concentration was increased to 10 mM (IPS-10 mM EGTA) Vm became significantly less negative: -13+/-2 mV (n=23, P<0.05). These results suggest that 10 mM EGTA reduces a calcium-dependent current involved in the maintenance of Vm. Depolarizing voltage steps up to +60 mV from holding potentials of -60 mV resulted in large (1-10 nA) time- and voltage-dependent outward currents. The amplitudes of total whole-cell current densities measured at voltages above -20 mV were significantly greater in the cells dialysed with IPS-0.1 mM EGTA than in those dialysed with IPS-10 mM EGTA. In the cells dialysed with IPS-0.1 mM EGTA, 0.1 mM tetraethylammonium chloride (TEA) and 50 nM iberiotoxin (IBTX), which selectively block large conductance Ca2+-activated potassium channels (BKCa), diminished the total current recorded at +60 mV by 45+/-14% (P<0.05, n=5) and 50+/-6% (n=8, P<0.05), respectively. These blockers at the same concentrations did not affect the total current in cells dialysed with IPS-10 mM EGTA. When tested on intact HSV rings, both 0.1 mM TEA and 50 nM IBTX elicited vessel contraction. We conclude that BKCa channels present in HSV smooth muscle cells contribute to the maintenance of the Vm and sustain a significant portion of the total voltage-activated, outward current. Finally, BKCa channels appear to play a significant role in the regulation of HSV smooth muscle contractile activity.

Evidence for an electrogenic Na+-HCO3- symport in rat cardiac myocytes.

1. The perforated whole-cell configuration of patch clamp and the pH fluorescent indicator SNARF were used to determine the electrogenicity of the Na+-HCO3- cotransport in isolated rat ventricular myocytes. 2. Switching from Hepes buffer to HCO3- buffer at constant extracellular pH (pHo) hyperpolarized the resting membrane potential (RMP) by 2.9 +/- 0.4 mV (n = 9, P < 0.05). In the presence of HCO3-, the anion blocker SITS depolarized RMP by 2.6 +/- 0.5 mV (n = 5, P < 0.05). No HCO3--induced hyperpolarization was observed in the absence of extracellular Na+. The duration of the action potential measured at 50 % of repolarization time (APD50) was 29.2 +/- 6.1 % shorter in the presence of HCO3- than in its absence (n = 6, P < 0.05). 3. Quasi-steady-state currents were evoked by voltage-clamped ramps ranging from -130 to +30 mV, during 8 s. The development of a novel component of Na+-dependent and Cl--independent steady-state outward current was observed in the presence of HCO3-. The reversal potential (Erev) of the Na+-HCO3- cotransport current (INa,Bic) was measured at four different levels of extracellular Na+. A HCO3-:Na+ ratio compatible with a stoichiometry of 2:1 was detected. INa,Bic was also studied in isolation in standard whole-cell experiments. Under these conditions, INa,Bic reversed at -96.4 +/- 1.9 mV (n = 5), being consistent with the influx of 2 HCO3- ions per Na+ ion through the Na+-HCO3- cotransporter. 4. In the presence of external HCO3-, after 10 min of depolarizing the membrane potential (Em) with 45 mM extracellular K+, a significant intracellular alkalinization was detected (0.09 +/- 0. 03 pH units; n = 5, P < 0.05). No changes in pHi were observed when the myocytes were pre-treated with the anion blocker DIDS (0.001 +/- 0.024 pH units; n = 5, n.s.), or when exposed to Na+-free solutions (0.003 +/- 0.037 pH units; n = 6, n.s.). 5. The above results allow us to conclude that the cardiac Na+-HCO3- cotransport is electrogenic and has an influence on RMP and APD of rat ventricular cells.

Beta-adrenoceptor activation and PKA regulate delayed rectifier K+ channels of vascular smooth muscle cells.

Macroscopic 4-aminopyridine (4-AP)-sensitive, delayed rectifier K+ current of vascular smooth muscle cells is increased during beta-adrenoceptor activation with isoproterenol via a signal transduction pathway involving adenylyl cyclase and cAMP-dependent protein kinase (PKA) (Aiello, E. A., M. P. Walsh, and W. C. Cole. Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H926-H934, 1995.). In this study, we identified the single delayed rectifier K+ (KDR) channel(s) of rabbit portal vein myocytes affected by treatment with isoproterenol or the catalytic subunit of PKA. 4-AP-sensitive KDR channels of 15.3 +/- 0.6 pS (n = 5) and 14.8 +/- 0.6 pS (n = 5) conductance, respectively, were observed in inside-out (I-O) and cell-attached (C-A) membrane patches in symmetrical KCl recording conditions. The kinetics of activation (time constant of 10.7 +/- 3. 02 ms) and inactivation (fast and slow time constants of 0.3 and 2.5 s, respectively) of ensemble currents produced by these channels mimicked those reported for inactivating, 4-AP-sensitive whole cell KDR current of vascular myocytes. Under control conditions, the open probability (NPo) of KDR channels of C-A membrane patches at -40 mV was 0.014 +/- 0.005 (n = 8). Treatment with 1 microM isoproterenol caused a significant, approximately threefold increase in NPo to 0. 041 +/- 0.02 (P < 0.05). KDR channels of I-O patches exhibited rundown after approximately 5 min, which was not affected by ATP (5 mM) in the bath solution. Treatment with the purified catalytic subunit of PKA (50 nM; 5 mM ATP) restored KDR channel activity and caused NPo to increase from 0.011 +/- 0.003 to 0.138 +/- 0.03 (P < 0. 05; n = 11). These data indicate that small-conductance, 15-pS KDR channels are responsible for inactivating the macroscopic delayed rectifier K+ current of rabbit portal vein myocytes and that the activity of these channels is enhanced by a signal transduction mechanism involving beta-adrenoceptors and phosphorylation by PKA at a membrane potential consistent with that observed in the myocytes in situ.

Uptake and release of Ca2+ in chemically skinned aortic strips from spontaneously hypertensive (SHR) and normotensive (WKY) rats.

The uptake and release of Ca2+ were studied in EGTA-skinned aortic strips from spontaneously hypertensive rats (SHR strain: SAP = 191 +/- 5 mmHg, n = 27) and normotensive control rats (WKY strain: SAP = 131 +/- 2 mmHg, n = 25). 45Ca uptake was measured as a function of time (0.5 to 30 min.), at pCa 6.6, in the presence of 10 mM of K oxalate. Skinned aortic strips of SHRs accumulated more Ca2+ after 30 min of uptake than those of WKY rats (0.66 +/- 0.05 vs 0.52 +/- 0.03 nmole.mg-1 wet tissue; p < 0.05). A lower activity of the transport system in the hypertensive group was evidenced by the fraction of these maximal uptake values accumulated after 2 minutes of uptake, 56% compared with 98% in the normotensive group. 45Ca release was assayed in skinned aortic strips preloaded for 30 minutes with 45Ca in the absence of K oxalate and desaturated with washing solutions containing 3 nM free Ca2+. 30 mM of caffeine, 5 microM of norepinephrine or 10 microM of IP3 resulted in greater increases in the rates of Ca2+ efflux in WKY than in SHR aortic strips. Net effluxes of Ca2+ upon stimulation with all these drugs were statistically significant only in the hypertensive group due to its slightly but consistently higher Ca2+ content. Changes in both rate of efflux and net efflux induced by 30 mM of caffeine could be blocked by 0.6 mM of ryanodine. The sarcoplasmic reticulum is characterized in the genetically hypertensive rats by a low transport activity of its Ca(2+)-ATPase, a high Ca2+ content and a Ca2+ release mechanism with low responsiveness to stimulation by caffeine, norepinephrine and IP3.

Calcium-induced calcium release in EGTA-skinned aortic strips from genetically hypertensive and normotensive rats.

The release of 45Ca induced by Ca2+ was studied in genetically hypertensive (SHR) and normotensive (WKY) rats using EGTA-skinned aortic strips. Strips preloaded with 45Ca (pCa 6.6) were desaturated at 5 mM of EGTA. A slow component of the washout in aorta from SHRs exhibited higher Ca content and a lower rate of Ca leak than that in aorta from WKY rats. This slow component was stimulated during washing with 0.03, 0.3, 1 or 10 microM free Ca2+. The release of 45Ca induced by Ca2+ proceeded at similar rates in preparations of the two strains. Compared to WKY aortic strips the stimulated efflux of 45Ca was greater in SHR aortic strips due to the higher Ca content. About half of the release of 45Ca induced by 1 microM free Ca2+ during the first 6 minutes of stimulation was blocked by 0.6 mM of ryanodine or 50 microM of ruthenium red, thus identifying the sarcoplasmic reticulum as a source of Ca release. The results suggest that this intracellular storage of Ca in aorta from genetically hypertensive rats is relevant for the generation of high levels of cytosolic Ca2+.

Transport and release of calcium by sarcoplasmic reticulum in chemically skinned ventricular muscle of the rat.

Strips of rat ventricle were treated with EGTA (5 mM) for 24 h at 4 degrees C and used to perform isotopical measurements of transport and release of Ca2+ in the SR in situ. 45Ca accumulated by these preparations showed dependence on time of incubation until it reached saturation after 30 min. Rate and capacity of Ca2+ accumulation were calculated in 0.075 nmol mg ww-1 min-1 and 0.402 nmol mg ww-1, respectively. These values were increased by a factor of 2.6 and 8.6 when K oxalate was present in the incubation media as could be expected for Ca2+ transported by SR. Ca2+ release was assayed on 45Ca desaturation curves at three free Ca2+ concentrations: 0.3, 1 and 10 microM. Significant increases in the velocity of Ca2+ efflux and net release of Ca2+ were induced only by 1 microM free Ca2+, and the Ca2+ release could be inhibited by 75% when 50 microM of ruthenium red was included in the washout solution. These results are in agreement with those obtained in assessing the SR function by mechanical measurements in skinned cardiac cells or by biochemical determinations in isolated cardiac SR vesicles. In spite of the fact that the resolution time is not as high as that required for the physiological handling of Ca2+ by SR, this methodology looks promising for approaching the SR function in cardiac pathologies as well as the effects of drugs on transport and release of Ca2+ by cardiac SR.

Comparison of sarcoplasmic reticulum characteristics in aortic smooth and ventricular muscles using chemically skinned preparations.

The active Ca2+ transport and the kinetics of Ca2+ efflux in sarcoplasmic reticulum (SR) of skinned aortic smooth muscle cells are compared with those in SR of skinned ventricular muscle cells. 45Ca uptake was measured at pCa 6.6 in the presence of 10 mM of K oxalate from 2 to 30 min. Unstimulated and Ca2+ stimulated Ca2+ efflux were assessed on Ca2+ desaturation curves performed in skinned cells preloaded with 45Ca in the absence of K oxalate. Results demonstrate that SR in aortic smooth muscle cells compared with SR in ventricular cells has: a) 2 times lower initial phase of Ca2+ uptake, b) 10 times lower enclosed volume, c) higher rate of unstimulated Ca2+ efflux and d) higher plus more sustained release of Ca2+ induced by Ca2+. It is also proved that the stimulated release of Ca2+ can be suppressed by 0.6 mM of ryanodine in aortic smooth muscle and by 50 microM of ruthenium red in ventricular muscle. This work provides direct measurements of Ca2+ transport capacity and of Ca2+ release of in situ sarcoplasmic reticulum. It quantifies the functional differences between vascular smooth and ventricular muscles.

[Calcium accumulation in hyperpermeable fibers of vascular smooth, skeletal and cardiac muscle of rats]. / Acumulación de calcio en fibras hiperpermeabilizadas de músculo liso vascular, esquelético y cardíaco de rata.

This work presents results on calcium accumulation capacity of chemically skinned multifiber preparations of aortic smooth, skeletal and ventricular muscle of the rat. The suppression of the plasma membrane functions as a permeability barrier to ions, allowed to expose the sarcoplasmic reticulum (SR) to 45Ca-EGTA buffers and to measure the amount of 45Ca accumulated by the preparations in a 30-min period at room temperature. The 45Ca loaded was attributed to the activity of the SR calcium pump since it was dependent on pCa values of the incubation media and enhanced by K oxalate. This method allowed to discriminate calcium accumulation capacity of SR in the different muscle types. 45Ca accumulated in the absence of K oxalate amounted to 5.16 +/- 0.08, 7.02 +/- 0.38 and 2.59 +/- 0.15 mumoles Ca2+/g fiber protein in aortic smooth, skeletal and ventricular muscle preparations, respectively. The values obtained in the presence of 10 mM K oxalate indicated the following increasing order of calcium accumulation capacities: skeletal greater than ventricular greater than or equal to aortic smooth muscle. Taking into account the protein content of each muscle type, the calculated amount of calcium accumulated by the SR exceeds that necessary to elicit full contractile activation.

Acumulación de calcio en fibras hiperpermeabilizadas de músculo liso vascular, esquelético y cardíaco de rata. / [Calcium accumulation in hyperpermeable fibers of vascular smooth, skeletal and cardiac muscle of rats]

This work presents results on calcium accumulation capacity of chemically skinned multifiber preparations of aortic smooth, skeletal and ventricular muscle of the rat. The suppression of the plasma membrane functions as a permeability barrier to ions, allowed to expose the sarcoplasmic reticulum (SR) to 45Ca-EGTA buffers and to measure the amount of 45Ca accumulated by the preparations in a 30-min period at room temperature. The 45Ca loaded was attributed to the activity of the SR calcium pump since it was dependent on pCa values of the incubation media and enhanced by K oxalate. This method allowed to discriminate calcium accumulation capacity of SR in the different muscle types. 45Ca accumulated in the absence of K oxalate amounted to 5.16 +/- 0.08, 7.02 +/- 0.38 and 2.59 +/- 0.15 mumoles Ca2+/g fiber protein in aortic smooth, skeletal and ventricular muscle preparations, respectively. The values obtained in the presence of 10 mM K oxalate indicated the following increasing order of calcium accumulation capacities: skeletal greater than ventricular greater than or equal to aortic smooth muscle. Taking into account the protein content of each muscle type, the calculated amount of calcium accumulated by the SR exceeds that necessary to elicit full contractile activation.