Non-selective cationic channels (NSCC) in smooth muscle cells from human umbilical arteries.

The aim of our work was to investigate the presence of non-selective cation channels (NSCC) in freshly isolated smooth muscle cells from the human umbilical artery (HUA), one of the vessels involved in fetal-placental circulation. We studied the electrophysiological properties of NSCC using the patch-clamp technique in whole-cell configuration, and their possible role in the contractile state of intact vessels' rings. Recording with a high intracellular Cs(+) solution and a near physiological extracellular saline solution, we found a Gd(3+)-sensitive current (IC(50) = 1.05 microM) with a linear current-voltage relationship showing a reversal potential (E(rev)) of -2.1 +/- 1.2 mV (n =15 cells). La(3+) (100 microM) and Mg(2+) (5 mM) also blocked this current. In such conditions, inward currents were carried by Na(+) and Ca(2+); hence, a Na(+)-free solution inhibited only inward current (-67.3 +/- 11.4%, at -40 mV, n = 7, p < 0.05) and a Ca(2+)-Na(+)-free solution decreased the current even further with respect to values obtained in Na(+)-free solution (-69.8 +/- 8.8% at -40 mV, n = 9, p < 0.05). The permeability ratios (P(X)/P(Cs(+))) for monovalent and divalent cations were 1, 0.9, 0.7, 0.7, 0.7, and 0.5 where X = Cs(+), Na(+), Li(+), Ca(2+), Ba(2+) and Tris(+), respectively. In intact tissue, a 0 Ca(2+) extracellular solution, Gd(3+) (100-250 microM), La(3+) (200 microM) and Mg(2+) (5 mM) induced vasorelaxation in non-stimulated HUA rings.

Influence of calcitonin gene-related peptide release on pH-induced mechanical depression in rat atria.

Rat atria is richly innervated by sensory nerve fibers that release CGRP when stimulated either by capsaicin or acid pH. We studied the physiological relevance of acid pH-induced CGRP release on changes in atrial contractility and relaxation produced by lowering the pH. Isolated atria electrically paced at 2.77 Hz were exposed to a 10-minute period of metabolic acidosis (pH=6.73+/-0.01, n=28) after: 1) CGRP release induced by capsaicin 0.5 microM; 2) blockage of CGRP release with ruthenium red (RR) 5 microM; 3) no pretreatment; and 4) CGRP receptor blockage with CGRP(8-37) 1 microM. Contractility and relaxation were significantly less depressed by acid pH when CGRP release was prevented by RR or CGRP receptor activation was blocked by CGRP(8-37). The results suggest that CGRP release and the activation of CGRP receptors may be physiologically involved in contributing to the depression of contractility and relaxation induced by acid pH in rat atria.

[Structural and functional aspects of the human saphenous vein used as aorto-coronary graft in myocardial revascularization]. / Aspectos estructurales y funcionales de la vena safena humana utilizada como puente aorto-coronario en la cirugia de revascularización miocárdica.

The human saphenous vein (HSV) is currently used as a graft in coronary revascularization as well as in some other vascular beds, namely those of the inferior limbs. Since a significant proportion of HSV grafts develop stenosis, many studies have focused on the factors that could promote graft failure. This article reviews the results on structural and functional features that might be concurrent in the production of saphenous vein graft stenosis. The reactivity of HSV to several physiological agonists is analyzed, including those derived from the endothelium with contractile or relaxing properties, since these are relevant inducers of graft spasm and/or modifiers of the expression of graft factors involved in either tissue growth or thrombotic-atherosclerotic processes. Mechanisms that regulate vascular smooth muscle contractile state, in particular the activity of K+ channels of the plasma membrane, are described.

Voltage-activated sodium current is inhibited by capsaicin in rat atrial myocytes.

The effects of capsaicin, the active principle of hot pepper genus Capsicum, were studied on voltage-activated, tetrodotoxin-sensitive Na+ currents in isolated rat atrial cells using the patch clamp technique in the whole-cell configuration. 0.4 and 4 microM of capsaicin produced a significant tonic block on voltage-activated Na+ current (I(Na)) evoked by a depolarizing step to -40 mV from a holding potential of -100 mV (49 +/- 7% n = 11, P < 0.05 and 72 +/- 13% n = 4, P < 0.05 respectively). We didn't observe any use-dependent block of capsaicin in our experimental conditions. Capsaicin slowed the time decay of inactivation of I(Na), and increased the time constant of the recovery of inactivation. Capsaicin and tetrodotoxin (TTX) depressed contractility of isolated electrically driven left rat atria, being the depression of maximal velocity of force development (dF/dt(max)) with respect to control values of 19 +/- 3% at 1 microM of capsaicin and 22 +/- 2% at 1 microM of TTX. These results show an inhibitory effect of capsaicin on I(Na) in isolated atrial cells that may modify the electrical and contractile function of the rat heart.

Insulin preincubation effects on rat vessel contractile responses: role of the endothelium.

The effect of contractions elicited with ET1 and AVP after preincubating rat aortic and tail artery rings with a hyperinsulinemic dose (3 nM) of insulin were studied. Insulin preincubation (120 min), in the presence of 0.1 mM L-NAME, depressed contraction of aortic rings to 0.01 microM ET1 (132 +/- 6 vs. 161 +/- 9 mg/mm2 in control, n = 25; p < 0.05) and to 1 microM AVP (84 +/- 7 vs. 110 +/- 9 mg/mm2 in control, n = 16; p < 0.05), but did not modify 45Ca influx to the cell. Insulin-induced relaxation was inhibited by indomethacin 10 microM, an antagonist of prostaglandin synthesis, and also by blockade of insulin receptors with 30 microM genistein. A short insulin preincubation (15 min) did not modify ET1 contractions. In rat tail artery, insulin preincubation (120 min) increased the force developed by ET1 (847 +/- 45 vs. 596 +/- 99 mgF/mgW in controls, n = 14) by stimulating TXA2 release and/or actions. In summary, the present results suggest that endothelial factors are involved in both the vasoconstrictor and vasodilator effects of insulin on rat vessels.

Insulin preincubation effects on rat vessel contractile responses: role of the sarcoplasmic reticulum.

In the present work, we studied the possible mechanisms involved in the insulin-induced acceleration of ET1 contractions. We observed a shortening of the half-life needed to achieve maximal developed force (t(1/2)) at 10(-7) M ET1 in rat aortic rings preincubated for 120 min with 3 nM insulin (control 380 +/- 15 s vs. 319 +/- 8 s with insulin, n = 28, p < 0.05). A tyrosine kinase linked receptor was involved in this effect because it was abolished by 30 microM genistein. Endothelium denudation and 10 microM indomethacin treatment did not effect this insulin effect, suggesting its independence of endothelial-derived factors. The effect was still present when the only source of Ca2+ was intracellular (t(1/2) values in the absence of external Ca2+: control 467 +/- 68 s vs. 213 +/- 28 s with insulin, n = 16, p < 0.05), but was blunted if the sarcoplasmic reticulum (SR) Ca2+ source was suppressed by exposure to 10 microM thapsigargin or 10 microM ryanodine. Preincubation with insulin did not potentiate either SR 45Ca2+ uptake or contractions evoked by caffeine-sensitive SR Ca2+ release. Since 30 microM cheleritrine abolished insulin-induced acceleration of ET1 contractions, we propose that the hormone might enhance a signal pathway related to PKC in order to produce a faster Ca2+ release from the SR.

Aspectos estructurales y funcionales de la vena safena humana utilizada como puente aorto-coronario en la cirugia de revascularización miocárdica. / [Structural and functional aspects of the human saphenous vein used as aorto-coronary graft in myocardial revascularization]

The human saphenous vein (HSV) is currently used as a graft in coronary revascularization as well as in some other vascular beds, namely those of the inferior limbs. Since a significant proportion of HSV grafts develop stenosis, many studies have focused on the factors that could promote graft failure. This article reviews the results on structural and functional features that might be concurrent in the production of saphenous vein graft stenosis. The reactivity of HSV to several physiological agonists is analyzed, including those derived from the endothelium with contractile or relaxing properties, since these are relevant inducers of graft spasm and/or modifiers of the expression of graft factors involved in either tissue growth or thrombotic-atherosclerotic processes. Mechanisms that regulate vascular smooth muscle contractile state, in particular the activity of K+ channels of the plasma membrane, are described.

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.

Role of insulin preincubation in the contractile reactivity of rat aortic rings.

Preincubation with physiological concentrations of insulin affects contractile reactivity of isolated smooth muscle cells. We studied the effects of insulin on intact aortic rings of Wistar rats preincubated 1-2 h with 240 pM (I1) and 960 pM (I2) insulin with and without NO synthesis inhibition by N(omega)-nitro-L-arginine methyl ester (L-NAME). Resting force was tripled by 0.1 mM L-NAME in control (C) and I1 groups, but not in I2 groups. I1 treatment decreased the tachyphylaxis to two successive 1 microM arginine vasopressin (AVP) stimulations. Single contractions elicited by 1 microM AVP, 1 microM angiotensin II (AngII), or 0.01 microM endothelin (ET1) were not affected by insulin preincubation in either maximal force (Fmax) or relaxation times. L-NAME enhanced Fmax of AngII contractions by about 75% in C, 120% in I1, and 74% in I2 groups; accordingly, it augmented the final steady-state force in C and I1 but not in I2. Similarly, L-NAME increased Fmax (30-40%) of AVP and ET1 contractions in C and I1 groups but failed to do so in contractions of I2 group. Results obtained with 10 microM indomethacin suggest that this is due to insulin stimulation of prostacyclin effects.

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+.

Interactive effects of variations in [Na]o and [Ca]o on rat atrial spontaneous frequency.

The effects of varying the extracellular concentrations of Na and Ca ([Na]o and [Ca]o) on both, the spontaneous beating and the negative chronotropic action of verapamil, were studied in the isolated rat atria. Basal frequency (BF) evaluated by surface electrogram was 223 +/- 4 beats/min. in control Krebs-Ringer containing 137 mM Na and 1.35 mM Ca (N). It decreased by 16 +/- 3% by lowering [Na]o to 78 mM (LNa), 23 +/- 2% by lowering simultaneously [Na]o to 78 mM and [Ca]o to 0.675 mM (LNa+LCa) and 31 +/- 5% by lowering [Na]o to 78 mM plus increasing [Ca]o to 3.6 mM (LNa+HCa). At normal [Na]o, decrease (0.675 mM) or increase (3.6 mM) of [Ca]o did not modify BF; a reduction of ten times (0.135 mM of normal [Ca]o was effective to reduce BF by 40 +/- 13%. All negative chronotropic effects were BF-dependent. Dose-dependent bradycardia induced by verapamil was potentiated by LNa, LCa, and HCa. Independent but not additive effects of Na and Ca are shown by decreases in the values of [verapamil]o needed to reduce BF by 30% (IC30) with the following order of inhibitory potency: LNa > LCa > HCa > N, resulting LNa+HCa similar to LNa. The [verapamil]o that arrested atrial beating (AC) was also potentiated with the order LNa = LNa+LCa = LNa+HCa = LCa > HCa = N. The results indicate that rat atrial spontaneous beating is more dependent on [Na]o than on [Ca]o in a range of +/- 50% of their normal concentration. Also the enhancement of verapamil effects on atrial beating was more pronounced at LNa than at LCa.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between systolic arterial pressure and heart mass in the rat.

This study describes the relationship between left ventricular mass and systolic arterial pressure (SAP) in normotensive and hypertensive rats. The genetically hypertensive rats (SHR) were stratified according to their SAP values in to hypertensive (SHRH) and normotensive (SHRN) subgroups. Males and females of both subgroups exhibited biventricular hypertrophy and the left ventricular mass was not correlated with SAP values. An overall cardiac enlargement was also found in female rats of the normotensive WKY group. In DOCA-salt and 2K/1C hypertensive rats the left ventricular mass was correlated positively with SAP. These data suggest that SAP is not a factor in the development of left ventricular hypertrophy in the SHR strain, however, they also revealed a right ventricular mass increase in this strain with respect to the Wistar strain.

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.

The effect of pH on the calcium dependence of calcium accumulation in dog cardiac muscle sarcoplasmic reticulum.

Net Ca2+ accumulation in vesicles of dog cardiac sarcoplasmic reticulum (CSR) was evaluated at three different pHs: 6.0, 6.8 and 7.6. The Ca2+ sequestration by CSR depends on Ca2+ concentration and on pH values. The curves that show the relationship between Ca2+ accumulated by CSR and external Ca2+ concentrations were shifted with pH changes, both in the absence and in the presence of potassium oxalate. Considering the curve at pH 6.8 as reference, a lower Ca concentration was needed to obtain the half-maximal value in Ca sequestration under pH 7.6 (0.04 +/- 0.006 and 0.79 +/- 0.09 microM at pH 7.6 and 6.8, respectively). Opposite results were obtained under pH 6.0 (13.66 +/- 1.29 microM). Net calcium release during active accumulation of Ca2+ and Ca2+ efflux from passively 45Ca2+ loaded CSR microsomes were significantly higher at alkaline pH than at acidic pH. The results suggest that in CSR alkaline pH would promote the increase in the rates of both, Ca2+ release and active Ca2+ accumulation, while opposite effects would be expected under acidic pH. Therefore, pH changes may regulate both, the Ca2+ level upon which the SR Ca2+ pump works (permeability effect) and the sequestration rate of the Ca2+ pump (variation in the affinity for calcium).

Evaluation of right and left ventricular size in SHR-Wistar hybrids.

The relative right and left ventricular sizes were evaluated in hybrid rats obtained by crossing spontaneously hypertensive (SHR) and pure Wistar strains to detect cardiac hypertrophy dissociated from the hypertension genetically transmitted by the SHR strain. In female hybrids of the the F1, F2, and F3 generations, both ventricles were found to have bigger size in relation to body weight than in the pure Wistar, whereas only the F3 group was hypertensive. All generations of male hybrids had arterial pressure values in the hypertensive range, which were not consistently accompanied by increases in the ventricle mass. The correlation between arterial pressure and indexes of ventricular size was assessed in sex-age matched groups. In a range of systolic pressure values of 100-200 mmHg, the variables showed no correlation or poor positive correlation (correlation coefficient values from -0.3641 to 0.6153). The correlation was not improved in the F3 generation as would be expected, because these hybrids underwent higher left pressure load than the preceding generations. The results indicate that increased ventricular size in SHR-Wistar hybrids may be independent from the hypertension genetically transmitted by the SHR strain and suggest that some of the previously proposed factors, i.e., cardioadrenergic activity or the growth factor isolated from SHR hearts, may be playing a role in the ventricular hypertrophy process in this strain. If this characteristic of hybrids is a constant in pure SHR strain, its validity as a model of cardiac hypertrophy due to left pressure overload would be questioned.

Cardiac sarcoplasmic reticulum characteristics in hypertrophic hearts from spontaneously hypertensive rats.

Sarcoplasmic reticulum (SR) from left ventricles of rats that developed spontaneous hypertension was studied in vitro. Similar increases of left ventricular mass were found when grouping the animals into mild and severe hypertensives (average systolic arterial pressure of 168 +/- 4 and 202 +/- 6 mmHg, respectively). The amount of SR protein (mg/g of left ventricle) was higher when obtained from hypertrophic ventricles of both hypertensive groups than from ventricles of the control group. The result agreed with the enhanced Ca2+ uptake exhibited by left ventricular homogenates of the hypertensive groups. Consequently, Ca2+ uptake in SR microsomes isolated per gram of left ventricle (nmol Ca2+/g muscle) was 51.62 +/- 10.06 and 64.99 +/- 12.84 in mildly and severely hypertensive groups vs. 17.37 +/- 5.79 in the control group (P less than 0.05). The SR microsomes obtained from ventricles of hypertensive rats showed an enhanced Ca2+ activated ATPase activity that was not accompanied by increased Ca2+ uptake at saturating calcium concentrations, but by increased affinity for calcium (K'app. of 1.09 +/- 0.28 and 2.67 +/- 0.16 microM in SR microsomes of hypertrophic and control ventricles respectively; P less than 0.05). The rates of calcium loss, measured in SR vesicles passively loaded with 45Ca, were similar when assayed in SR obtained from ventricles of both hypertensive and normal rats. These results enable us to suggest that in hearts of rats presenting spontaneous hypertension, the function of the SR system could account for a normal handling of cytosolic calcium. They might support the absence of mechanical alterations described in hearts of young rats of the SHR strain.