Myocardial contractile actions of endothelin-1 in rat and rabbit papillary muscles. Role of endocardial endothelium

The actions of endothelin on the contractile and twitch configuration characteristics of rat and rabbit papillary muscles were evaluated before and after endocardial endothelium removal. In rabbit papillary muscles, endothelin produced a dose-dependent (10(-11) to 10(-7) M) increase in tension (T) (from 2.7 +/- 0.2 to 8.9 +/- 0.7 g/mm2, p less than 0.01), dT/dt, and Vmax (from 1.14 +/- 0.03 to 3.0 +/- 0.13 Lmax/sec, p less than 0.01), a decrease in time to peak tension (from 251 +/- 9 to 216 +/- 7 msec, p less than 0.05), and an increase in time to half relaxation (from 168 +/- 9 to 293 +/- 14 msec, p less than 0.01). Increasing calcium concentration in the bath from 1.25 to 15 mM greatly attenuated these changes. In the presence of propranolol (10(-5) M), endothelin increased all indexes of contractility but to a lesser extent (T = 2.6 +/- 0.3 to 6.5 +/- 0.2 g/mm2, p less than 0.01; Vmax = 1.16 +/- 0.10 to 2.06 +/- 0.10 Lmax/sec, p less than 0.01). Myocardial catecholamine depletion with reserpine had effects similar to those of propranolol. The myocardial contractile and twitch configuration characteristics of endothelin were similar to those of phenylephrine (10(-4) M), a strong stimulator of the phosphatidylinositol pathway and a moderate beta-adrenergic stimulator. The effects of phenylephrine and endothelin were modified in a similar manner by propranolol (10(-5) M). The presence of nicardipine (3 X 10(-7) M) decreased the absolute increase in contractility caused by endothelin but did not alter the percent change or shift the dose-response curve of endothelin. The actions of endothelin in rat papillary muscles studied at physiological calcium concentrations (Ca2+ = 1.25 mM) were less marked than those of rabbit studied at physiological calcium (Ca2+ = 1.25 mM) but similar to those of rabbit studied at high calcium concentrations (Ca2+ = 15 mM). Removing the endocardial endothelial layer of rabbit papillary muscles did not alter the absolute changes in contractility caused by endothelin but shifted the dose-response curve to the left and markedly altered the effects of endothelin on twitch configuration characteristics. Thus, it would appear that endothelin increases contractility and modifies twitch configuration. It does so more at lower rather than higher extracellular calcium concentrations and in a species, such as rabbit, that responds more to interventions that increase intracellular calcium.(ABSTRACT TRUNCATED AT 400 WORDS)     

The relative sensitization by acidosis of five calcium blockers in cat papillary muscles

We have previously reported that the negative inotropic effects of both verapamil and nifedipine on cat papillary muscles are enhanced as pH is lowered from 7.4 to 6.8 and 6.0. These studies have now been extended to compare the relative sensitization by acidosis of verapamil, nifedipine, lidoflazine, perhexilene and diltiazem. Developed tension was recorded in cat papillary muscles and the calcium concentration was adjusted over the range 2 to 10 mM. At pH 7.4, addition of all five drugs moved the dose response curve to the right with pA2 values from 4.82 (lidoflazine) to 9.94 (nifedipine). At pH 6.0, there was eight-fold sensitization by acidosis for verapamil, but four, three, and two-fold sensitization for nifedipine, lidoflazine and perhexilene. Diltiazem, however, was not sensitized by acidosis. The differential effects of acidosis on the negative inotropic properties of the five drugs may reflect their ancillary properties opposite gating of the calcium channel, local anaesthesia, intracellular calcium movement or Na+/Ca2+ exchange, but also suggest that diltiazem may have the property of inhibiting the effects of low pH on cell membranes.     

Faster time to peak tension and velocity of shortening in right versus left ventricular trabeculae and papillary muscles of dogs

Some of the mechanical characteristics of papillary muscles and trabeculae (n = 14) isolated from the free wall of the right ventricle of dogs were compared to those obtained from the free wall of the left ventricle (n = 14). Peak total tension (7.2 +/- 1.6 versus 7.9 +/- 1.7 g/mm2, p = NS) and peak velocity of tension development (31 +/- 8 vs. 28 +/- 4 g/mm2/sec, p = NS) were similar in both groups of muscles. However, right ventricular muscles shortened faster over nearly all loading conditions, and during isometric contraction, their time to attain peak total tension was shorter (336 +/- 26 vs. 401 +/- 42 msec, p less than 0.005) than that of left ventricular muscles. Varying stimulation rates (6, 12, 24, and 36 stimuli/min), increasing calcium concentration from 2.54 to 6.35 mM or adding norepinephrine 50 microM, did not significantly alter these differences. There were no differences in myosin isozymes (V1, V2, or V3) between ventricles to explain these differences. These results indicate that important mechanical differences exist between right and left ventricular myocardium and that these differences should be considered when extrapolations are made from myocardium of one area of the heart to another.     

Age-related differences in excitation-contraction coupling in rat papillary muscle

Summary To investigate the possible role of an alteration in excitation-contraction coupling during development, aging and senescence we compared simultaneously recorded mechanical and electrical activity of left ventricular papillary muscles from 3, 6, 12, and 24-month-old male rats. In addition, the effects of calcium and verapamil on excitation-contraction coupling were evaluated.We recorded transmembrane action potentials during both isometric and isotonic contractions. At an external bath calcium=2.4 mM, action potential duration at 75% complete repolarization (APD₇₅) was significantly prolonged as a function of age (3 mo=28.2±2.7; 6 mo=29.5±2.6; 12 mo=49.5±5.6; 24 mo=121±8.5 msec) while peak developed tension (DT) was not significantly altered (3 mo=5.13±0.53; 6 mo=4.75±0.53; 12 mo=7.26±0.51; 24 mo=6.01±0.67 g/mm²). The correlation coefficient (r value) for APD₇₅ and DT was strong for 3-month-old animals (r=0.99) but weakened as a function of age (6 mo=0.93; 12 mo=0.81; 24 mo=0.57). Similar results were observed when APD₇₅ was correlated with time-to-peak tension (TPT) (3 mo=0.95; 6 mo=0.98; 12 mo=0.85; 24 mo=0.68), time-to-one-half relaxation (T1/2R) (3 mo=0.91; 6 mo=0.97; 12 mo=0.85; 24 mo=0.81) and time to peak shortening (TPS) (3 mo=0.89; 6 mo=0.81; 12 mo=0.82; 24 mo=0.51). Correlations between action potential duration and contractile parameters became weak in all age groups upon the addition of verapamil (V). The correlation between APD₇₅ and DT for 3-month-old animals decreased by 34% upon the addition of V while a 70% decrease was seen in 24-month-old animals. Similar results were seen when APD₇₅ was correlated with TPT, T1/2R and TPS when V was added to the perfusate. Our results indicate that excitation-contraction coupling, as evidence by alterations in not only the contractile apparatus but also in the surface membrane, may be altered in ventricular muscle as in function of age.Support in part by NIH Grants # HL 21933-04, # 07071-06 and a Herman Raucher Investigatorship Award of the New York Heart Association to Dr. Joseph M. Capasso. Animals used in this study were generously awarded to Dr. Capasso by the Animal Acquisitions Branch of the National Institute of Aging.     

Effects of dynamic exercise on muscle function in elderly men, aged 70 years

The maximal electrically evoked and voluntary contractions of the triceps surae were measured, before and after 30 min of continuous uphill walking, in 8 elderly men aged 67-71 years. The exercise was performed on a motor-driven treadmill and corresponded to 66-74% of their predicted maximal aerobic power output. During the control period, before exercise, twitch tension (Pt), time-to-peak tension (TPT) and half relaxation time (1/2RT) of the maximal twitch response were 102 +/- 29 N, 156 +/- 15 ms and 98 +/- 11 ms; the tetanic responses at frequencies of 10 Hz (PO10) and 20 Hz (PO20) and the maximal voluntary contraction (MVC) averaged 600 +/- 103, 766 +/- 106 and 1,225 +/- 185 N, respectively. The fatigue index in response to a 2-min test involving repeated tetanisation of the triceps surae was 0.40 +/- 0.14. None of these variables (except TPT which was decreased to 145 +/- 11 ms, p less than 0.001) was significantly changed following exercise. It was concluded that exercise of relatively high intensity and duration in healthy elderly men does not impair the force-generating capacity of the lower leg muscles nor increase their fatigability.     

Exogenous coenzyme Q attenuates the tension prolongation phenomenon

In order to determine whether exogenous coenzyme Q can modify the course of mechanical changes in the myocardium during hypoxia and reoxygenation, we studied isolated rat heart muscles contracting isometrically at Lmax, 12 times per minutes at 28 degrees C. The prehypoxic mechanical parameters of 6 control muscles and 6 muscles treated with coenzyme Q10 (40 m micrograms/ml in the bath) did not differ, nor did the changes in developed tension and resting tension during 15 min of hypoxia and 45 min of reoxygenation. However, the time it took for the tension to fall to 50% of the peak value (RT1/2), the time from the onset of contraction to peak isometric tension (TPT) and TPT + RT1/2 on early reoxygenation were significantly diminished in coenzyme Q treated muscles. Thus, the tension prolongation phenomenon was attenuated in preparations of coenzyme Q treated rat heart muscle. It is concluded from this study that exogenous coenzyme Q does not modify the mechanical performance of isolated rat heart muscle during hypoxia but significantly attenuates the tension prolongation phenomenon. Since oxygen free radicals are known to induce mitochondrial dysfunction and impairment of calcium sequestration in the sarcoplasmic reticulum, the mechanism by which coenzyme Q attenuates the tension prolongation phenomenon is probably attributable to its function as an antioxidant.     

Endothelin-1-induced vasoconstriction in humans. Reversal by calcium channel blockade but not by nitrovasodilators or endothelium-derived relaxing factor.

The vascular effects of endothelin-1 (ET) in humans were investigated by brachial artery infusions of ET into 25 healthy volunteers. Forearm blood flow increased from a mean +/- SD value of 2.3 +/- 1.5 to 2.5 +/- 1.5 ml/min/100 ml forearm tissue (n = 25, p less than 0.05) in response to low dose (0.5 ng/min/100 ml forearm tissue) ET infusion and decreased to 1.78 +/- 1.3 and 1.1 +/- 0.9 ml/min/100 ml forearm tissue (p less than 0.001) during higher dosages (25 and 50 ng/min/100 ml forearm tissue). Sodium nitroprusside (0.6 micrograms/min/100 ml forearm tissue, n = 6), acetylcholine (16 micrograms/min/100 ml forearm tissue, n = 7), nifedipine (6 micrograms/min/100 ml forearm tissue, n = 6), and verapamil (80 micrograms/min/100 ml forearm tissue, n = 6) were infused alone and in combination with ET to evaluate the interactions between ET-induced vasoconstriction and stimulation of vascular muscle cyclic GMP levels by sodium nitroprusside, release of endothelium-derived relaxing factor by acetylcholine, and blockade of voltage-operated calcium channels by nifedipine and verapamil. Neither the vasodilator nor the vasoconstrictor response to ET was influenced by sodium nitroprusside or acetylcholine. In contrast, both calcium antagonists converted ET-induced vasoconstriction (e.g., delta forearm vascular resistance to ET 50 ng/min/100 ml forearm tissue, 151 +/- 100% and 164 +/- 92% in verapamil and nifedipine groups, respectively) to vasodilation (-35 +/- 12% and -21 +/- 16%, p less than 0.05). Our results demonstrate both ET-induced vasodilation (at low dosages) and vasoconstriction (at high dosages) in resistance vessels of normal humans. Blockade of voltage-operated calcium channels prevented ET-induced vasoconstriction and unmasked the vasodilator effect of high ET dosages. In human resistance vessels, blockade of voltage-operated Ca2+ channels but not cyclic GMP-dependent vasodilation may be an effective tool to inhibit ET-induced vasoconstriction.     

Systemic hypertension induces disparate localized left ventricular action potential lengthening and altered sensitivity to verapamil in left ventricular myocardium

Previously, we demonstrated that acute exposure to superfusate containing verapamil suppresses action potential plateau to a greater degree in cells at the tip of the anterior papillary muscle compared to those at its base in normal cat left ventricle (LV) studied in tissue bath. To determine the effects of chronic pressure overload brought about by renal hypertension on papillary muscle electrophysiology and regional sensitivity to verapamil, LV were isolated from cats subjected to unilateral nephrectomy and contralateral kidney wrapping. After 3 months, systemic hypertension (mean increase = 60.2 +/- 1.4 mmHg) was associated with moderate LV hypertrophy of approximately equal to 18% (LV weight/body weight). Transmembrane action potentials recorded from the endocardial surface (including anterior papillary muscle) in pressure overloaded LV in vitro (800 ms stimulus cycle length) showed increased action potential duration at 25, 75 and 90% of repolarization (APD25, APD75 and APD90, respectively) relative to controls (LV from normal and sham operated cats). With respect to the anterior papillary muscle in pressure overloaded LV, APD25, APD75 and APD90 were increased to a greater extent in cells at the base of the muscle compared to those increases observed at the tip (P less than 0.05). In contrast to its effects in controls, verapamil (2 micrograms/ml) significantly reduced APD25 at both the base and the tip of the papillary muscle in pressure overloaded preparations, but particularly at the base; also, the disparities between the APD75 or APD90 at tip and base were decreased. Thus, regional electrophysiologic disparities were induced in systemic hypertension, and these differences were subsequently reduced by verapamil.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of contractile state and myosin isozymes of rat right and left ventricular myocardium

We compared myocardial mechanics and myosin isozymes of right and left ventricular papillary muscles from adult (6 to 8 month old) male rats. Analysis of force velocity relations indicate that right ventricular papillary muscles contract more rapidly than left at light loads (2.68 +/- 0.13 vs 2.18 +/- 0.07 muscle lengths/s measured 75 ms following stimulation, at 0.5 g/mm2; P less than 0.01). Right ventricular papillary muscles had significantly more of the alpha heavy chain containing V1 myosin isozyme and less of the V3 containing beta heavy chain myosin isozyme than left ventricular preparations (P less than 0.05). Papillary muscle and ventricular free wall myosin isozyme distribution were not significantly different within their respective chambers. The presence of a relatively larger proportion of the alpha heavy chain containing myosin isozyme (V1) in right ventricle papillary muscles relative to left correlated with the more rapid velocities of shortening seen in right ventricular papillary muscles (r = 0.60; P less than 0.01).     

Arterial vasodilator effects of the dihydropyridine calcium antagonist amlodipine alone and in combination with verapamil in systemic hypertension

The arterial vasodilator properties of the dihydropyridine calcium antagonist amlodipine were compared with the effects of vascular muscle cyclic guanosine monophosphate production by sodium nitroprusside and with the effects of a combined infusion of amlodipine and the nondihydropyridine calcium antagonist verapamil in 8 untreated patients with primary hypertension. Arterial vasodilation was assessed by measurement of changes of forearm blood flow by mercury in Silastic strain-gauge plethysmography during brachial artery drug infusions. Forearm blood flow increased during amlodipine infusions (0.4 to 45 micrograms/min/100 ml forearm tissue) from 2.9 +/- 1.7 to a maximum of 23.6 +/- 7.6 ml/min/100 ml (687%), while sodium nitroprusside caused an increase from 3.0 +/- 1.8 to 16.2 +/- 5.4 ml/min/100 ml (449%), attesting to the importance of transmembrane calcium influx for the maintenance of vascular tone. The addition of verapamil 40 micrograms/min/100 ml to an infusion of amlodipine 44.5 micrograms/min/100 ml resulted in a further increase of forearm blood flow, from 23.6 +/- 7.6 to 34.4 +/- 9.8 ml/min/100 ml (p less than 0.05). The precise mechanisms of this finding have yet to be elucidated but may be due to interactions of the effects of the binding of these 2 chemically and pharmacologically different calcium antagonists to distinct binding sites at calcium channels. The clinical relevance of this observation for the treatment of coronary artery disease and systemic hypertension needs further study.     

Requirement of extracellular calcium in fish pituitary gonadotropin release by gonadotropin hormone-releasing hormone

Influence of extracellular calcium on gonadotropin hormone-releasing hormone (GnRH)-stimulated gonadotropin hormone (GtH) release from a teleostean fish (Channa punctatus) pituitary was examined in vitro by preparing enzymatically dispersed pituitary cell incubation. Effect of Ca2+ on GnRH-augmented GtH release was evaluated with partially purified C. punctatus GnRH (cGnRH) and synthetic mammalian GnRH (mGnRH). Cells were dispersed by 0.3% collagenase plus 0.05% trypsin in culture medium and a high yield of viable cells were obtained. Addition of cGnRH (10 micrograms/ml) to pituitary cells in Ca2+-free medium resulted in a significant increase in GtH release, but the addition of Ca2+ (2 mM) enhanced it to about four- and threefold over cGnRH and mGnRH, respectively. Increasing concentrations of Ca2+ (0.1-2.0 mM/well) with fixed concentrations of GnRH (10 micrograms/ml) or increasing doses of GnRH (2.5 to 20 micrograms/ml) with fixed amount of Ca2+ (2 mM/well) resulted in a dose dependent increase in GtH release. EDTA or EGTA (2 mM/well) completely suppressed the Ca2+-augmenting effect of GnRH-stimulated GtH release. Addition of lanthanum (La3+, 4 mM/well), a competitive inhibitor of Ca2+, reduced 60% of the Ca2+ (2 mM/well) stimulation. Verapamil, a specific Ca2+ channel blocker, when added in increasing concentrations (1-100 microM/well) to pituitary cell incubations containing GnRH-stimulated GtH release in Ca2+-free medium could be waived by EGTA (2 mM/well), indicating availability of extracellular calcium from tissue sources. The uptake of radioactive Ca2+ by pituitary cells was greatly enhanced by GnRH while the addition of verapamil (10 microM/well) not only inhibited the GnRH-stimulated uptake, but also reduced it below the control level.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of isoproterenol on twitch contraction of wild type and phospholamban-deficient murine ventricular myocardium

Ablation of the gene for phospholamban (PLB), a transmembrane peptide regulator for the cardiac sarcoplasmic reticulum Ca2+ pump, in mice brings about a complete loss of the myocardial responses to beta-adrenergic agonists (e.g., Luo et al., Circ. Res. 1994; 75: 401). We have evaluated the functional significance of PLB-independent mechanisms in the myocardial responses to beta-adrenergic stimulation in isolated intact ventricular myocardium. We compared the effects of (-)-isoproterenol (ISO) on isometric twitch contraction of paced right ventricular muscle strips of wild type (WT) and PLB-deficient (PLBKO) mice. At 37 degrees C, frequent spontaneous contractions in both types of muscles required the inclusion of lidocaine, an antiarrhythmic, in the bathing medium. Thus the experiments were also performed at two lower temperatures, 30 degrees C and 25 degrees C, at which lidocaine was not needed. Under three conditions, in the absence of ISO, PLBKO ventricular muscles exhibited substantially shortened time to peak tension (TPT) and half relaxation time (TR1/2), compared with the WT muscles. In both WT and PLBKO muscles ISO increased the peak developed tension and decreased TPT and TR1/2 in a dose-dependent manner although the effects were generally smaller in PLBKO than in WT muscles. One micromolar ISO caused TPT and TR1/2 to decrease by 7.3+/-1.2% (mean+/-SEM) and 7.5+/-1.2% in PLBKO vs. 22.8+/-0.7% and 29.1+/-1.7% in WT at 37 degrees C; by 13.5+/-0.4% and 14.1+/-1.2% in PLBKO vs. 31.3+/-0.8%, and 44.8+/-1.3% in WT at 30 degrees C; by 15.0+/-2.3% and 21.1+/-4.9% in PLBKO vs. 25.8+/-1.9% and 54.0+/-1.9% in WT at 25 degrees C. These findings strongly suggest that PLB-independent mechanisms play a significant role in mediating the positive inotropic and lusitropic effects of beta-adrenergic agonists on ventricular myocardium.     

Influence of gender and diabetes on vascular and myocardial contractile function.

Premenopausal women have a lower cardiovascular risk than men or postmenopausal women. However, this "female advantage" is lost in diabetes, a condition characterized by cardiac and vascular contractile dysfunction. This study was designed to compare the influence of diabetes on vascular and myocardial contractile function between genders. Adult male and female rats were made diabetic with streptozotocin (55 mg/kg) and maintained for 8 weeks. Tension development was examined in thoracic aortic rings and left ventricular papillary muscles. KCl-induced vasoconstriction, acetylcholine (ACh)-induced endothelium dependent or sodium nitroprusside (SNP)-induced endothelium-independent vasorelaxation, duration and maximal velocity of myocardial contraction and relaxation duration (TPT/RT90 and +/- VT) were similar between males and females. Diabetes augmented KCl-induced vasoconstriction at low doses, reduced SNP-induced vasorelaxation and had little effect on ACh-induced vasorelaxation in aortic rings from both genders. Diabetes prolonged TPT and RT90 in both genders, and reduced +/- VT in males but not females. Acute increase in extracellular Ca2+ (2.7 mM to 5.4 mM) shortened TPT in diabetic myocardium from both genders, whereas it had no effect on other myocardial mechanical indices in normal or diabetic groups of either gender. In addition, acute exposure to the Na+/K(+)-ATPase inhibitor ouabain shortened TPT/RT90 and enhanced +/- VT in myocardium from normal female, whereas it had no effect on male or diabetic myocardium. In conclusion, these data suggest that in the vasculature, there is no difference in diabetes-induced contractile dysfunction between genders, however several gender-specific differences are evident in the myocardium.     

Role of calcium in acute hyperthermic myocardial injury

INTRODUCTION: We hypothesized that intracellular calcium overload may play an important role in heat-induced myocardial injury. This postulate was investigated using a model of isolated guinea pig papillary muscle in which resting tension was measured as an indirect indicator of cytosolic free-calcium concentration and the fluorescence changes of Fluo-3 AM dye was measured as a direct indicator of cytosolic free-calcium concentration. METHODS AND RESULTS: Excised guinea pig right ventricular papillary muscles were attached to a force transducer in a high-flow tissue bath and superfused with Tyrode's solution at 37 degrees +/- 0.5 degrees C. The temperature was rapidly changed to between 38.0 degrees and 56.0 degrees C for 60 seconds and then returned to 37.0 degrees C. Hyperthermia caused a reversible increase in resting tension at temperatures between 45 degrees and 50 degrees C and irreversible contracture at > or =50 degrees C. Rapid cooling contracture experiments and experiments measuring fluorescence of myocytes loaded with 5 microM Fluo-3 AM dye demonstrated that the hyperthermia-induced rise in resting tension was likely due to an increase in intracellular calcium content. Inhibition of the sarcoplasmic reticulum calcium pump with 20 microM thapsigargin resulted in irreversible contracture of the papillary muscles at temperatures between 45 degrees and 50 degrees C and significant increases in Fluo-3 fluorescence at 48 degrees C. Blockade of sarcolemmal calcium channels with 0.5 mM cadmium or 40 microM verapamil did not attenuate the heat-induced increase in resting tension and Fluo-3 fluorescence. CONCLUSION: Hyperthermia causes an increase in resting tension of cardiac muscle that most likely is mediated by a calcium channel-independent increase in calcium permeability of the sarcolemmal membrane and/or release of stored intracellular calcium.     

Verapamil suppresses atherosclerosis in cholesterol-fed rabbits

The effect of verapamil, a drug that reduces the concentration of intracellular calcium, on atherogenesis was evaluated in rabbits fed a cholesterol-rich diet for 10 weeks. Ten rabbits received oral verapamil, 8 mg/kg daily; eight received the same oral dose and 0.5 mg/kg daily subcutaneously; nine received oral lanthanum, 35 mg/kg daily, and nine were controls. Over the 10 week period, all groups had average serum cholesterol levels greater than 1,500 mg/dl (normal = 90 +/- 63 mg/dl). At the end of the experiment, the aortas were removed, opened and stained for lipid with Sudan IV. The extent of atherosclerosis was determined by planimetry. The group receiving oral and parenteral verapamil had significantly less atherosclerosis (25 +/- 26% of total intimal area; mean +/- standard deviation), as compared with the controls (73 +/- 24%). Reduction of atherosclerosis with oral verapamil (51 +/- 22%) and lanthanum (59 +/- 31) was not statistically significant. Indexes of contractility in isolated right ventricular papillary muscles (developed tension at maximal length [Lmax] and maximal velocity of shortening [Vmax]) were reduced in the group treated with oral and parenteral verapamil, but not in the others. It is concluded that verapamil suppresses the development of atherosclerosis in rabbits fed a cholesterol-rich diet.     

Biphasic contractions induced by milrinone at low temperature in ferret ventricular muscle: role of the sarcoplasmic reticulum and transmembrane calcium influx

The effects of milrinone were studied in ferret papillary muscle stimulated at various rates and temperatures from 23 degrees to 36 degrees C. In voltage-clamp experiments, 50 micrograms/ml (0.237 mM) milrinone induced a 2.1-fold increase in calcium current at 28 degrees or 36 degrees C. At 50 micrograms/ml, milrinone transiently increased contractility in all muscles at 28 degrees C, but its steady-state effect was either increased (+50%) or decreased (-24.7%) steady-state twitch amplitude. A negative inotropic effect always occurred below 27 degrees C. Milrinone decreased the total twitch duration and split the twitch into two components (P1 and P2) in the absence of any evidence of aberrant conduction. Increasing milrinone concentration from 50 to 300 micrograms/ml decreased P1 and increased P2. Ryanodine (100 mM) or caffeine (10 mM) suppressed P1. Contractions elicited after 30 seconds of rest were also biphasic in the presence of milrinone, but not in its absence. P2 of post-rest contraction was increased by caffeine or calcium (10 mM) and decreased by cobalt (2 mM) when drugs were applied at the beginning of the rest. Ryanodine and caffeine also suppressed P1 of post-rest contraction. The evidence suggests that P1 may be caused by Ca release from the sarcoplasmic reticulum and P2 by increased Ca influx during the action potential via the calcium channel. It is also suggested that P2 may be present under control conditions, but to a lesser extent, and masked by a large P1.     

Sex differences in myocardial contractility in the rat

Summary We examined the intrinsic contractile performance of papillary muscles removed from the left ventricle of male and female Wistar rats. Muscles were studied isometrically and isotonically, stimulated at 0.1 Hz, perfused with Tyrode's solution having an external calcium concentration=2.4 and maintained at 30°C. In addition, we examined muscle response to changes in external calcium, added norepinephrine or verapamil and alterations in contraction frequency. No significant change in peak isometric development tension was observed between male and female preparations. However, muscles from male rats showed a significantly greater isometric time-to-peak tension and time to 1/2 relaxation with a depression of both the maximum rate of tension rise and maximum rate of tension decay. Isotonically, although peak shortening showed no difference between male and female preparations, the maximum velocities of shortening and relaxation were significantly depressed in muscles from male rats. Muscles from male animals also displayed significant prolongation of the time-to-peak shortening and time-to-peak velocity of shortening. These differences in papillary muscle performance were found over a wide range of muscle lengths, stimulus frequencies and bath concentrations of calcium, norepinephrine and verapamil. Thus differences in intrinsic contractile performance between papillary muscle from male and female rats have been characterized.Supported in part by NIH grants #HL 21933-02, #HL 07071-05 and a Herman Raucher Investigatorship Award of the New York Heart Association to Dr. J. M. Capasso.     

The economy of isometric force development, myosin isoenzyme pattern and myofibrillar ATPase activity in normal and hypothyroid rat myocardium

Hypothyroidism was induced in Wistar-Kyoto rats by adding propylthiouracil to the drinking water (0.8 mg/ml). Initial heat, total activity-related heat, and resting heat rate were measured in left ventricular papillary muscle preparations of propylthiouracil-treated and control rats contracting isometrically at 12 beats/min (21 degrees C), using Hill type, planar vacuum-deposited bismuth and antimony thermopiles. In the propylthiouracil preparations, relative to control, time-to-peak tension increased from 288 +/- 27 (mean +/- SD) to 411 +/- 25 msec (P less than 0.001), dp/dtmax decreased from 38.3 +/- 9.5 to 20.4 +/- 3.5 g X mm-2/sec (P less than 0.001), and peak developed tension decreased from 6.11 +/- 1.75 to 4.64 +/- 0.89 g X mm-2 (P less than 0.05). In the propylthiouracil preparations, initial heat was significantly (P less than 0.001) reduced by 27 or 43% when normalized to peak twitch tension or tension-time integral, respectively. In experiments where the papillary muscles were tetanized, the slope of the linear function of total activity-related heat versus tension-time integral was decreased by 43% (P less than 0.001) in the propylthiouracil preparations, indicating an improved economy of isometric tension maintenance. The predominant myosin isoenzyme of the left ventricular wall, as well as the papillary muscle myocardium, was the V3 variety in the propylthiouracil animals, in contrast to V1 in the controls. Myofibrillar actomyosin calcium-magnesium-stimulated adenosine triphosphatase activity was significantly (P less than 0.02) decreased from 55 +/- 18 (control) to 31 +/- 8 nmol inorganic phosphate ion/mg X min (propylthiouracil).(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in intracellular Na+ during Na,K pump inhibition in sheep cardiac tissues.

The differences in inotropic and toxic sensitivities of cardiac ventricular tissue to cardiac glycosides were investigated in order to determine whether or not the reportedly greater sensitivity of Purkinje fibers compared to myocardium is the result of a fundamentally different response to Na,K pump inhibition. I measured the changes in intracellular Na+ activity (aiNa) in the two tissue types simultaneously during exposure to actodigin (4.0 microM) and ouabain (0.5 microM) under quiescent conditions. A sheep papillary muscle and a Purkinje fiber from the same heart were placed in an experimental chamber and measurements of aiNa from both were obtained with Na+ -sensitive microelectrodes. In five experiments in which all electrode impalements were successfully maintained, actodigin caused similar changes in aiNa in the two tissues (from 7.2 +/- 1.0 mM in control to 12.9 +/- 1.9 mM in the presence of drug in papillary muscles compared to 7.3 +/- 0.3 mM in control and 13.2 +/- 1.0 mM in Purkinje fibers; means +/- S.E.M.). After washout, exposure to ouabain increased aiNa in both papillary muscles and in Purkinje fibers (from 7.2 +/- 0.7 mM in control and 16.2 +/- 1.4 mM during exposure to drug in papillary muscles compared to 7.4 +/- 0.3 mM and 14.9 +/- 0.8 mM in Purkinje fibers). In fact, ouabain caused a greater increase of aiNa in papillary muscles than in Purkinje fibers (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Modulation of the positive inotropic effect of pyruvate by energetic substrate availability

Dependence of pyruvate's positive inotropic effect on energetic substrate availability and potential role of its mitochondrial uptake were investigated. Pyruvate (3, 10 and 15 mM) was added to rabbit right ventricular papillary muscles (protocol I; n = 10) and human right auricular trabeculae (protocol II; n = 6), using glucose as energetic substrate. In protocols III & IV (rabbit papillary muscles; n = 8 and n = 10, respectively) pyruvate's mitochondrial uptake was inhibited by alpha-cyano-4-hydroxycinnamate (0.5 mM) with octanoate as energetic substrate at 5 and 0.2 mM, respectively. In 8 additional rabbit papillary muscles, effects of L-alanine (10, 20 and 50 mM) were tested. In protocols I&II, pyruvate had a dose-dependent positive inotropic effect that was maximal at 10 mM, increasing in rabbit myocardium: 45.0+/-9.4% active tension, 20.5+/-7.4% peak rate of tension rise, 32.5+/-8.6% peak isotonic shortening, 31.2+/-11.7% peak rate of lengthening, 27.8+/-3.2% twitch duration. In protocol III (5 mM octanoate), pyruvate's positive inotropic effect was still present and even enhanced, while in protocol IV (0.2 mM octanoate) it was decreased and not observed with 3 mM of pyruvate. We conclude that, in rabbit papillary muscles, the positive inotropic effect of pyruvate is modulated by the availability of metabolic substrates and presumably does not depend on its mitochondrial uptake.