Rat Ventricular Muscle Research Articles

Aging: changes in cardiac α1-adrenoceptor responsiveness and

Several investigators have reported a diminished responsiveness of senescent cardiac muscle to norepinephrine and β-adrenoceptor agonists. In contrast, relatively little is known regarding the effects of aging on myocardial actions mediated specifically by α-adrenoceptor stimulation. Thus, the current study examined aging-dependent changes in: (a) the inotropic response to methoxamine, and α-adrenoceptor agonist; (b) characteristics of myocardial α 1-adrenoceptors as monitored by specific [ 3H]prazosin binding; and (c) steady state levels of α 1-adrenoceptor mRNA as determined by Northern blot analysis. Cardiac preparations were isolated from 4-, 14-, and 25-month-old F344 rats. An aging-associated decline was observed in the maximum positive inotropic effect elicited by methoxamine in right ventricular strips (160 ± 23, 134 ± 13 and 79 ± 26% increase above control developed tension in 4, 14 and 25 months, respectively) with no change in ED 50 values. [ 3H]Prazosin binding to ventricular sarcolemmal membranes revealed a reduction in receptor number (82 ± 7, 69 ± 6 and 59 ± 5 fmol/mg protein in 4, 14 and 25 months, respectively); the apparent dissociation constant was not affected. Steady state levels of α 1-adrenoceptor mRNA decreased progressively between 4 and 25 months of age (14- and 25-month levels were approximately 71 and 38% of 4 months, respectively), while steady state levels of β-actin mRNA did not change with age. These results suggest that the aging-related decline in α 1-adrenergic responsiveness in rat ventricular muscle is mediated, at least in part, by a decrease in cardiac α 1-adrenoceptor density. In turn, the decline in receptor number may be a direct result of diminished levels of α 1-adrenoceptor gene transcripts in aging myocardium.

Frequency- and length-dependent tension development in rat heart muscles exposed to isoflurane and halothane

Using 22 isolated rat ventricular muscle preparations, we investigated whether or not the increase in preload and/or contraction frequency may counteract the negative inotropy of both isoflurane (2.0%) and halothane (1.0%). Increases in preload from 94% of Lmax (the length where muscles produce the maximal tension) to Lmax did not alter significantly the percent decrements in tension development caused by either isoflurane or halothane. The increases in contraction frequency from 0.1 to 0.6 Hz augmented the depressant effect of isoflurane significantly ( P < 0.001), while the depressant effect of halothane was not altered at these contraction frequencies. Small but significant counteraction occurred in the depressant effects of halothane at 0.8 and 1.6 Hz ( P = 0.002). These changes in intracellular mechanism(s), resulted from the increase in contraction frequency, interacted with the two anesthetics on tension development, while these may not be the case for the increase in preload.

Myocardial alpha 1-adrenoceptors mediate positive inotropic effect and changes in phosphatidylinositol metabolism. Species differences in receptor distribution and the intracellular coupling process in mammalian ventricular myocardium.

Species-dependent variations of myocardial alpha 1-adrenoceptor-mediated positive inotropic effects of epinephrine were assessed in relation to characteristics of alpha 1-receptor bindings and acceleration of phosphatidylinositol metabolism in the isolated rat, rabbit, and dog ventricular myocardium. Epinephrine in the presence of the beta-adrenoceptor antagonist bupranolol (10(-6) M) elicited a positive inotropic effect through activation of alpha 1-adrenoceptors in rat and rabbit, whereas in dog ventricular myocardium, bupranolol abolished the positive inotropic effect of epinephrine. [3H]Prazosin bound to membrane fractions derived from rat, rabbit, and dog ventricular muscle with high affinities in a saturable and reversible manner. In dog, Bmax and Kd values of alpha 1-adrenoceptor binding sites were identical to those in rabbit ventricular muscle. The Bmax value of alpha 1-adrenoceptors in rat ventricle was the highest, amounting to two to four times those in rabbit and dog. Epinephrine displacement curves for the specific binding of [3H]prazosin in the membrane fraction of these species showed high and low affinity sites with slope factors significantly less than unity, which were shifted to single low affinity sites with slope factors close to unity by addition of 5'-guanylylimidodiphosphate. Accumulation of [3H]inositol 1-phosphate [( 3H]IP1) in ventricular slices prelabeled with [3H]myo-inositol was increased by epinephrine in a time- and concentration-dependent manner in rat ventricular slices. [3H]IP1 accumulation likewise was facilitated by alpha 1-adrenoceptor stimulation in rabbit ventricular slices, whereas the extent of [3H]IP1 accumulation was much less than that in rat. In dog ventricular slices, [3H]IP1 was not accumulated by epinephrine. In rabbit papillary muscle, the time course of increase in contractile force induced by alpha-adrenoceptors coincided with the prolongation of the action potential duration with a similar time course, which is in strong contrast to previous findings in rat that the contractile response was dissociated from the electrophysiological response to alpha-adrenoceptor stimulation. The present results indicate that a wide range of variation of alpha 1-adrenoceptor-mediated regulation of myocardial contractility may be ascribed to different contributions of facilitatory as well as inhibitory regulatory processes that lead to intracellular Ca2+ mobilization subsequent to myocardial alpha 1-adrenoceptor activation among mammalian species.

Temperature effects on the Na and Ca currents in rat and hedgehog ventricular muscle

Cardiac transmembrane potentials and Na and Ca currents were recorded at different temperatures in rat and hedgehog ventricular muscle. At 35 °C in both species resting potential was about −80 mV and upstroke velocity ( V ̇ max ) of the action potential above 100 V s . The shape of the action p in hedgehog ventricular cells at 35 °C was similar to that in the rat showing a fast repolarization phase. When temperature was decreased, the membrane resting potential depolarized and action potential amplitude and V ̇ max declined. In rat ventricular cells at 10 °C, the resting potential was about −40 to −50 mV and V ̇ max was reduced to about 5 V s . In hedgehog ventricular cells, however, the tra potentials and V ̇ max were better maintained at low temperature. Phase 3 of the action potential was markedly prolonged below 20 °C in hedgehog but not in rat ventricular cells. When temperature was decreased to 10 °C the availability curve of the Na current shifted toward more negative potentials and I Ca,peak declined in rat ventricular cells. In hedgehog cardiac preparations, the Na current was less influenced by the cooling and I Ca,peak did not change very much at low temperatures. A transient inward current usually considered to induce cardiac arrhythmias could be recorded in rat ventricular cells below 20 °C but not in hedgehog preparations. These features of hedgehog cardiac membranes may contribute to the cold tolerance and the resistance to ventricular fibrillation during the hypothermia in mammalian hibernators.

Influence of experimental diabetes on sarcoplasmic reticulum function in rat ventricular muscle

We examined whether the decrease in cardiac contractility in streptozotocin-induced diabetes in the rat is accompanied by reduced or excessive loading of the sarcoplasmic reticulum (SR) with Ca2+. Pooled SR Ca2+ content and fractional release on stimulation were estimated with rapid cooling contracture (RCC) and twitch height measurements, respectively. Interval-force relation was studied to assess the ability of diabetic tissue to alter the relative contribution of SR Ca2+ for contraction. Two months after injection with streptozotocin, peak isometric contraction and steady-state RCC decreased in parallel to approximately 50% of control values. The time to peak force development and complete relaxation was prolonged to 156 and 161% in diabetes in the presence of 1.25 and 2.5 mM extracellular Ca2+ concentration [Ca2+]o, respectively. A stepwise increase in the rate of stimulation from 0.2 to 0.5 and 1.0 Hz resulted in a negative force staircase, the slope of which was identical in control and diabetic animals in each [Ca2+]o tested. Postrest contractions and RCC, after variable test intervals, were significantly depressed after 0.2 and 0.5 Hz stimulation in diabetic muscles at 1.25 mM [Ca2+]o. This defect of SR Ca2+ availability was reversed by increasing the stimulation frequency to 1.0 Hz or by elevating [Ca2+]o to 2.5 mM. The results suggest that the marked reduction of developed tension in diabetic tissues was a consequence of depleted SR Ca2+ stores, rather than a result of chronic SR Ca2+ overloading. The maintained integrity of the interval-force relation in the presence of diabetes implies that the cellular mechanisms responsible for frequency- and time-dependent alterations in SR Ca2+ availability are not disturbed at this stage of disease.

Effect of excitability-inducing material on regenerative Ca++release of rat ventricular muscle

The effects of excitability-inducing materials (EIM) and electrical stimulation on Ca+(+)-induced Ca++ release were studied in this experiment. Direct application of three EIM (bacterial EIM, alamethicin, Nal) did not release Ca++ from skinned rat cardiac fibers or from an SR suspension. Direct electrical stimulation of the SR suspension had no effect on Ca++ release either. However, both electrical stimulation and pre-treatment with an EIM augmented Ca+(+)-induced Ca++ release in the skinned fiber and led to increased contractility of the fiber in a dose-dependent manner. When electrical stimulation and EIM application were superimposed, their effects were additive, suggesting that information transfer across the junction between T-tubule and SR is at least chemical in nature.

Modulation of stimulation frequency responses and calcium dependency of functional parameters in hyperthyroid rat ventricular papillary muscles

The effects of stimulation frequency (0.2-1.5 Hz) and extracellular calcium concentration ([Ca2+]o) (0.6-15.0 mM) on the contractile function of thin papillary muscles of euthyroid and hyperthyroid rats were studied. Hyperthyroidism led to a decrease in developed tension (DT) and time to peak tension (TPT), but it exhibited no influence on the maximal rates of contraction (+dT/dt) and relaxation (-dT/dt). Also, the mean rates of contraction were similar in euthyroid and hyperthyroid muscle groups. The increase in stimulation frequency brought about a marked decrease in DT, +dT/dt, and -dT/dt of euthyroid papillary muscles at lower frequencies in comparison to papillary muscles in the hyperthyroid group. At stimulation frequencies above 1.0 Hz, the absolute and relative levels of DT and -dT/dt of hyperthyroid myocardium were elevated over euthyroid preparations. At the same time, TPT was unchanged in any of the muscle groups. Hyperthyroidism modulated the relationships between contractile parameters and [Ca2+]o. At a [Ca2+]o of 1.0-4.0 mM, the DT of hyperthyroid papillary muscles was lower than in euthyroid muscle. At 4.0 and 8.0 mM of [Ca2+]o, the equal values of maximal DT were registered for euthyroid and hyperthyroid papillary muscles, respectively. An increase in the [Ca2+]o in the range of 1.0-15.0 mM was accompanied by an increase in TPT of both muscle groups, but to a greater extent in hyperthyroid myocardium. In conclusion, the myocardium of hyperthyroid rat appeared to exhibit decreased sensitivity to calcium as well as to the negative inotropic effect of enhanced stimulation frequency. Alterations of the processes of transsarcolemmal movement and intracellular recycling of Ca2 may be implicated.

Effects of taurine depletion on intrinsic contractility of rat ventricular papillary muscles

Contractile parameters in Krebs-Henseleit media containing various calcium concentrations were compared in left ventricular papillary muscles of two groups of rats: control and taurine depleted. All tests were carried out with the muscles at initial length, Lmax, the length that produced maximal active tension. From measurements of after- and un-loaded contractions, the velocity-tension curves and the derived maximum velocity of shortening were not different between the groups. Time to peak shortening and extent of shortening were not altered, while relaxation times and contraction duration were significantly prolonged for taurine-depleted muscles. Peak isometric tension and its rate of development were significantly reduced in taurine-depleted muscles compared with controls. Postrest (3 min) stimuli and paired stimuli (200-ms interval) evoked similar potentiated contractile responses in both groups, such that the ratio of their peak tensions remained unchanged. For taurine-depleted muscles the force-frequency relationship (a negative staircase) was parallel to, but lower than, control. These experiments suggest that taurine deficiency leads to reductions in action potential triggered calcium release from internal stores, and deficits in calcium sequestration. This may result from disfacilitation of calcium binding to the sarcoplasmic reticulum and other storage sites during taurine deficiency.

The cell surface of isolated cardiac myocytes—A light microscope study with use of fluorochrome-coupled lectins

In cells isolated from guinea-pig or rat ventricular muscle occurrence and distribution of carbohydrate components of the surface coat were monitored using fluorochrome-coupled lectins. Fluorescence of membrane-bound lectins was assayed by an image analysis system. The lectins ConA † † Abbreviations: BSA, bovine serum albumin; ConA, Concanavalin A; DBA, Dolichos biflorus agglutinin; DMSO, dimethyl sulfoxide; EGTA, ethyleneglycol-bis-(β-aminoethyl ether)- N,N,N′,N′-tetraacetic acid; FITC, fluorescein isothiocyanate; Gal, d-galactose; GalNAc, N-acetyl- d-galactosamine; Glc, d-glucose; GlcNAc, N-acetyl- d-glucosamine; HEPES, N-(2-hydroxyethyl)piperazine- N′-(2-ethanesulfonic acid); LFA, Limax flavus agglutinin; Man, d-mannose; met-Glc, methyl- d-glucoside; met-Man, methyl- d-mannoside; NeuNAc, N-acetylneuraminic acid; NeuNGc, N-glycoloylneuraminic acid; PNA, peanut agglutinin; RCA-I, Ricinus communis agglutinin-I; SBA, soy bean agglutinin; TMA-DPH, 1-[4(trimethylamino)phenyl]-6-phenyl-1,3,5-hexatriene; TRITC, tetramethylrhodamine isothiocyanate; UEA-I, Ulex europaeus agglutinin-I; WGA, wheat germ agglutinin; sWGA, succinylated WGA. , WGA, sWGA, LFA and RCA-I showed specific binding to the whole myocyte surface, indicating a homogeneous distribution of α-mannosyl, α-glycosyl, N-acetylglucosaminyl, N-acetylneuraminate and β-galactosyl residues. Binding of DBA and SBA, with specific affinity for N-acetylgalactosaminyl residues, to guinea-pig cardiac myocytes was mainly at the cell poles corresponding to intercalated discs in intact tissue. Both lectins failed to interact with rat myocytes. UEA-I, specific for α- l-fucose, bound slightly to rat and not to guinea-pig myocytes. Binding of PNA to guinea-pig myocytes was observed only after cleaving off sialic acids from cell surface, suggesting that sialic acids mask galactosyl-β(1,3)- N-acetylgalactosamine residues. Specifity of lectin-cell interaction was tested by an inhibition assay where free sugars were tested for their capacity to inhibit lectin binding to the myocytes. When comparing different isolation procedures based on different proteolytic enzymes, the myocytes' affinity to any lectin was found to be qualitatively unchanged. Investigation of lectin-decorated myocytes by means of confocal laser scan microscopy showed that lectin binding sites are not confined to the cell surface but are also present in sarcolemmal invaginations, i.e. transverse tubules. This suggests that the tubular system is lined with a carbohydrate layer similar to, and continuous with, that of the peripheral cell surface.

Functional interconversion of rest decay and ryanodine effects in rabbit and rat ventricle depends on Na/Ca exchange

Rapid cooling contractures were used to assess changes in sarcoplasmic reticulum (SR) Ca content in isolated rabbit and rat ventricular muscle during rest, with altered transsarcolemmal [Na] and [Ca] gradients and in the presence and absence of 100 nM ryanodine. In rabbit there is normally a rest-duration dependent decline in SR Ca content (rest decay), whereas in rat there is a short-term increase in SR Ca content (rest potentiation) and little evidence of rest decay. Ryanodine greatly accelerates the rate of rest decay in rabbit, depleting the SR of Ca in approximately 1 s, whereas in rat, ryanodine does not appear to drain the SR even after a 10 min rest. Elevation of intracellular Na activity in rabbit (by Na-pump inhibition) to a level similar to that measured in control rat during rest (Shattock and Bers, Am. F. Physiol., 256: C813-C822, 1989) makes rest-dependent changes of SR Ca content in these two tissues similar. The rest decay in rabbit in the presence of ryanodine is also markedly slowed after Na-pump inhibition. In rat, reduction of [Ca]0 allows rest decay to occur (+/- ryanodine), but this rest decay can be largely prevented by simultaneous reduction of [Na]o (to maintain [Na]3/[ Ca] constant) which serves to keep the thermodynamic driving force on a 3:1 Na/Ca exchange constant. We conclude that the process of rest decay and rest potentiation in both rabbit and rat ventricle depends on the sarcolemmal Na/Ca exchange. Furthermore, these species can be functionally interconverted by manipulation of the [Na] and [Ca] gradients. The ability of ryanodine to deplete the SR of Ca also depends critically on other transport systems (particularly Na/Ca exchange) to remove Ca from the cytoplasm.

Chronic diabetes mellitus prolongs action potential duration of rat ventricular muscles: circumstantial evidence for impaired Ca2+ channel

The aim of the study was to investigate the effects of chronic diabetes mellitus on electromechanical properties of ventricular papillary muscles. Conventional glass microelectrodes and tension recording techniques were used in isolated hearts of rats made diabetic for 30-40 weeks by single intravenous injections of streptozotocin. Experimental animals were male Wistar rats of 200-250 g. Diabetic rats (n = 14) were given streptozotocin 65 mg.kg-1; controls (n = 15) were given vehicle only. (1) The maximum upstroke velocity of the action potential duration of diabetic muscles was decreased compared to control, with no difference in the resting potential. (2) At all stimulation frequencies (0.2, 1 and 5 Hz), and particularly the lower ones, the action potential duration of diabetic muscles was longer than control. (3) In diabetic muscles, frequency dependent shortening of the late phase of action potential duration (APD75, APD90) was more pronounced, and frequency dependent lengthening of the early phase (APD25, APD50) was less pronounced. (4) A blocker of transient outward current, 4-aminopyridine, lengthened the early phase of action potential durations by the same amount in diabetic and control muscles. (5) A Ca2+ channel blocker, CoCl2, dramatically shortened all levels of action potential duration, with much greater effect on diabetic muscles. (6) Ryanodine lengthened the early phase of action potential duration and shortened the late phase in both diabetic and control muscles. It enhanced the difference between the groups in the early phase. (7) Developed tension in the presence of ryanodine (ryanodine resistant tension component) was greater in diabetic muscles than in control. The findings suggest that altered Ca2+ current, but not altered Na(+)-Ca2+ exchange current or altered transient outward current, significantly prolongs action potential duration in diabetic rat ventricular muscles.

Two subtypes of dihydropyridine-sensitive calcium channels in rat ventricular muscle

Two opposite inotropic effects of the dihydropyridine activators, CGP 28392 and Bay K 8644, given at the same concentration (1–2 μM) were found in rat papillary muscles: a positive effect in polarized tissue (4 mM KCl) and a negative one during partial depolarization. The depressive effect found at a low rate or after a short rest was associated with marked prolongation of the Ca 2+-mediated action potential, indicating that the drugs behave as Ca channel stimulators. The depressive effect of the activation on the resting state contraction was antagonized by nifedipine (2 μM) and high Mg 2+ (5 mM). It was suggested that at least two subtypes of the L-type, dihydropyridine-sensitive channels underlie the opposite inotropic responses of the activators. The positive effect is apparently caused by conventional stimulation of Ca 2+ entry through the cell membrane, whereas the negative effect is probably due to the stimulation of Ca 2+ efflux from the sarcoplasmic reticulum, leading to depletion of intracellular stores. The effect was proposed to be mediated by activation of junctional channels linked to sarcoplasmic reticulum Ca 2+ release. An important role for these channels in triggering the sarcoplasmic reticulum Ca 2+ release and regulation of force-frequency relation is proposed.

Effect of resting muscle length on the force-interval relationship in rat ventricular muscle

Effect of resting muscle length on the force-interval relationship in rat ventricular muscle

Temperature dependence of myofilament Ca sensitivity of rat, guinea pig, and frog ventricular muscle

Cooling the superfusate of intact ventricular muscle, from 30 degrees C to below 4 degrees C in less than 2 s, leads to contractures thought to reflect the amount of Ca available for release from the sarcoplasmic reticulum (SR). On rewarming, tension transiently increases in guinea pig and rat ventricular muscle. It has been proposed that this rewarming tension spike reflects changes in myofilament Ca sensitivity and maximum Ca-activated force (Cmax) associated with rewarming. There are differences in intracellular Ca regulation among cardiac muscle preparations. Some characteristics of rapid-cooling contractures (e.g., the magnitude of the rewarming spike) also differ between species. Therefore, the Ca sensitivity of skinned ventricular muscle from the rat, guinea pig, and frog was determined at 29 (22 degrees C for frog ventricular preparations), 8, and 1 degrees C. The results show that cooling rat and guinea pig ventricular muscle from 29 to 1 degrees C shifts the pCa vs. tension relationship toward higher [Ca2+] by 0.65 and 0.55 pCa units, respectively. Cooling to 1 degrees C also reduced Cmax to 3.3 and 7.8% of that at 29 degrees C in rat and guinea pig ventricular muscle, respectively. Similar results were found for frog ventricular muscle, in which cooling from 22 to 1 degrees C reduced Ca sensitivity by 0.6 pCa units and Cmax to 45.7% of its value at 22 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes in β‐adrenoceptors, muscarinic cholinoceptors and Ca2+ channels in rat ventricular muscles

1. In an attempt to explain the previous electrophysiological data on the ontogeny of beta-adrenergic and muscarinic cholinergic interactions on cardiac Ca2+ current, biochemical studies were performed on the ontogeny of beta-adrenoceptors, muscarinic cholinoceptors and Ca2+ channels in cardiac muscle of developing rats: 16-20 days old foetuses, 0-20 days old neonates, and 2-3 months old adults. 2. Developmental changes in cardiac beta-adrenoceptors, muscarinic cholinoceptors, and Ca2+ channels were determined with the use of specific radioligands, [3H]-dihydroalprenolol (DNA), [3H]-quinuclidinyl benzilate (QNB), and [3H]-nitrendipine (NTD), respectively. 3. The Bmax value (fmol mg-1 tissue) for [3H]-DNA binding started to increase on post-gestation day 20, reached almost its maximum level on neonatal day 6, kept almost the same level until neonatal day 20, and then decreased slightly to its adult level. 4. The Bmax value (fmol mg-1 tissue) for [3H]-QNB binding started to increase on post-gestation day 16, reached almost its maximum level on neonatal day 0, remained almost constant until neonatal day 15, and then decreased to its adult level. 5. The Bmax value (fmol mg-1 tissue) for [3H]-NTD binding increased with age between post-gestation day 18 and neonatal day 15, stayed almost constant until neonatal day 20, and then decreased to its adult level. 6. The Kd values for [3H]-DHA, [3H]-QNB, and [3H]-NTD bindings remained almost constant during the developmental period examined. 7. Isoprenaline (Iso) increased the kx of slow action potentials (APs) from post-gestation day 18, and the adult level was reached at about 2 weeks after birth; this developmental time course is similar to that of Ca2+ channels. The number of beta-adrenoceptors also started to increase a few days before birth, but attained its peak about one week earlier than did the Pax of slow APs or the number of Ca2 + channels. 8. Acetylcholine (ACh) almost completely abolished the Iso-induced increase in m,,ax observed from postgestation day 18 to neonatal day 20; this developmental time course for the ACh effect is consistent with the finding that the number of muscarinic cholinoceptors started to increase on post-gestation day 16 and reached a peak on the day of birth. 9. Previous electrophysiological and the present biochemical findings strongly suggest that the functional coupling between muscarinic cholinoceptors and Ca2+ current is already established when the coupling between beta-adrenoceptors and Ca2 + current starts to operate in developing rat hearts.

Developmental changes in β‐adrenergic and cholinergic interactions on calcium‐dependent slow action potentials in rat ventricular muscles

1. Developmental changes in the effect of isoprenaline (Iso) and acetylcholine (ACh) interactions on Ca2(+)-dependent slow action potentials (APs) were studied in the ventricular muscles of foetal (12-20 days post-gestation), neonatal (0-20 days old), and adult (2-3 months old) rats. The slow APs were recorded at 0.2 Hz in partially depolarized preparations (an extracellular K+ concentration of 25 mM). 2. Iso (1 nM to 10 microM) began to increase the Vmax of the slow APs (an approximate indicator of Ca2+ current) on foetal day 18; its potentiating effect became greater with age and reached the adult level about 2 weeks after birth. 3. ACh (10 microM) abolished the Iso (1 microM)-induced increased in the Vmax observed in the late foetal and neonatal periods. 4. The inhibitory effect of ACh on the Vmax was antagonized by atropine but not by pirenzepine, suggesting that ACh reduces Ca2+ current (in the presence of beta-adrenoceptor agonists) by stimulating muscarinic (M2) cholinoceptors. 5. These results suggest that developmental changes in the modulatory effects of beta-adrenoceptor and cholinoceptor agonists on Ca2+ channels occur from a few days before birth to 2 weeks after birth and that the functional coupling between muscarinic cholinoceptors and Ca2+ channels has already been established when the coupling between beta-adrenoceptors and Ca2+ channels starts to operate.

Differential response of cultured adult cardiac muscle cells to a tumor promotor: Analysis of myofibrillar organization

Cultured adult rat ventricular cardiac muscle cells were exposed to varying concentrations of 12-0-tetradecanoyl-phorbol-13 acetate (TPA) for two weeks. A considerable number of cardiac myocytes exposed to a medium with less than 200 ng/ml TPA were rich in myofibrils. The rest of the myocytes lacked organized myofibrils; the terminal parts of these myofibrils were transformed into cord-like structures largely consisting of dense Z-band materials. Some of these aberrant myofibrils contained short normal myofibrillar segments, with sarcomeres. A number of myocytes exposed to 200–250 ng/ml TPA contained myofibrils. which terminated in cord-like structures. The Z-band materials appeared as amorphous dense matrices and some sarcomeres were replaced completely or partially by leptomeres; the myocytes contained autophagosomes. The other myocytes did not contain myofibrils when exposed to the above higher concentrations of TPA. The patches of Z-band materials and structures containing Z-band materials attached to thin filaments on either side were scattered throughout the sarcoplasm of the cells, which were packed with myofilaments and 10 nm microfilaments. Some of these myocytes assumed a spindle shape and contained myofilaments. 10 nm microfilaments and leptomeres. Some of the myocytes, treated with TPA for 1–7 days and then allowed to recover in control medium for 7 days, contained various stages of myofibrillar organization, which did not differ significantly from those of the myofibril-containing cells exposed continuously to TPA as discussed above. The rest of the myocytes during the recovery period in control medium did not contain myofibrils. Rough endoplasmic reticulum and Golgi bodies in TPA-treated myocytes were found to be highly developed as compared to the controls. It is evident from these studies that the responsiveness of cardiac myocytes to TPA not only differs from that of skeletal muscle cells studied by others, but also varies within a population of cardiac myocytes.

Ultrastructural morphometric analysis of cultured neonatal and adult rat ventricular cardiac muscle cells.

Neonatal and adult rat ventricular cardiac muscle cells cultured on laminin differed from similar myocytes grown on plastic in the amount and distribution of their mitochondria and transverse tubules. Point-count morphometry was used at the electron microscopic level to quantify these differences. Adult myocytes grown on laminin contained more mitochondria per unit volume than adult myocytes grown on plastic. No significant differences were observed in the volume percent of myofibrils in either adult or neonatal ventricular myocytes when grown on laminin and compared to those grown on plastic. The transverse tubule system in neonatal and adult myocytes was reduced significantly when both groups were cultured on laminin. Furthermore, neonatal and adult myocytes cultured on laminin were flatter than those cultured on plastic. This may indicate a relationship between the surface/volume ratio and transverse tubule development in cultured myocytes. These studies establish that point-count morphometry can be used to quantify changes in the organelle volume densities of cultured cardiac muscle cells.

Characterization of MS-857, a New and Potent Cardiotonic Agent, in Isolated Mammalian Cardiac Muscles

The positive inotropic activity of MS-857, a novel cardiotonic agent, was assessed in isolated atrial and ventricular muscle of several mammalian species. MS-857 (0.1-300 microM) increased the developed tension of the dog, rabbit, monkey, cat, and guinea pig ventricular muscles in a concentration-dependent manner. However, MS-857 failed to cause a positive inotropic effect in isolated rat ventricular muscle. In isolated guinea pig atria, MS-857 caused both positive inotropic and chronotropic effects. MS-857 was more selective for inotropy than for chronotropy as compared with either milrinone or 3-isobutyl-1-methylxanthine (IBMX). The positive inotropic and chronotropic effects of MS-857 do not appear to be mediated by direct stimulation of alpha- or beta-adrenoceptors, histamine H1- or H2-receptors, or by release of endogenous catecholamines. The inotropic response to isoproterenol or histamine was potentiated by a relatively low concentration of MS-857. MS-857 increased both developed tension and intracellular cyclic AMP level in isolated dog right ventricular muscle. The time course of the elevation in cyclic AMP level was paralleled by that of increase in developed tension. Carbachol inhibited the increase in developed tension and cyclic AMP level caused by MS-857. These findings support the idea that cyclic AMP may be involved in the positive inotropic effect of MS-857.

Effects of tolbutamide, glibenclamide and diazoxide upon action potentials recorded from rat ventricular muscle

Drugs which influence the electrical activity of insulin-secreting B cells of mammalian islets of Langerhans by closing (tolbutamide and glibenclamide) or opening (diazoxide) ATP-sensitive potassium channels were applied to the ventricular muscle of the rat. Action potentials were recorded from ventricular epicardium of perfused intact rat hearts. Tolbutamide (0.5-2.0 mM), glibenclamide (0.01-0.1 mM) and diazoxide (0.5 mM) each evoked a dose-dependent increase (7-33%) in the duration of the ventricular action potential measured at 50% of repolarization. These drugs were without effect upon the resting membrane potential or the peak of the action potential. Single-channel recordings of ATP-sensitive K+ channels were obtained from excised membrane patches of enzymatically isolated rat ventricular myocytes. Tolbutamide and diazoxide inhibited openings of ATP-sensitive K+ channels. Diazoxide inhibited ATP-sensitive K+ channel openings in the presence of ATP. Diazoxide did not evoke opening of ATP-sensitive K+ channels. It is concluded that these drugs could act to increase the duration of the cardiac action potential by inhibiting openings of ATP-sensitive K+ channels.