Positive force-and [Ca 2ϩ ] i -frequency relationships in rat ventricular trabeculae at physiological frequencies Downloaded from

Layland, Joanne, and Jonathan C. Kentish. Positive force-and [Ca 2ϩ ] i -frequency relationships in rat ventricular trabeculae at physiological frequencies. Am. J. Physiol. 276 (Heart Circ. Physiol. 45): H9-H18, 1999.-The isometric force-frequency relationship of isolated rat ventricular trabeculae (diameter Ͻ250 µm) was examined at 24, 30, and 37°C at stimulation frequencies (0.1-12 Hz) encompassing the physiological range. Some muscles were microinjected with fura PE3 to monitor the diastolic and systolic intracellular concentration of Ca 2ϩ ([Ca 2ϩ ] i ). At a near-physiological external Ca 2ϩ concentration ([Ca 2ϩ ] o ) of 1 mM, a positive force-frequency relationship was demonstrated at all temperatures. The force-frequency relationship became negative at high frequencies (e.g., Ͼ6 Hz at 30°C) at 1 mM [Ca 2ϩ ] o or at low frequencies at 8 mM [Ca 2ϩ ] o . The twitch and Ca 2ϩ transient became shorter as stimulation frequency increased; these changes were related to changes in systolic, rather than diastolic, [Ca 2ϩ ] i and were not blocked by inhibitors of Ca 2ϩ / calmodulin-dependent protein kinase II. The positive forcefrequency relationship of rat trabeculae was caused by a frequency-dependent loading of the sarcoplasmic reticulum (SR) with Ca 2ϩ . We suggest that at high frequencies, or under conditions of Ca 2ϩ overload, this loading saturates. Processes that tend to decrease SR Ca 2ϩ release will then predominate, resulting in a negative force-frequency relationship. fura PE3; calcium uptake by sarcoplasmic reticulum; calcium/ calmodulin-dependent protein kinase IN ISOLATED MYOCARDIAL preparations from most mammalian species an increase in the frequency of stimulation produces an increase of steady-state twitch force. This positive force-frequency relationship is generally attributed to a net uptake of Ca 2ϩ into cells as the frequency of stimulation is increased (26). The Ca 2ϩ loading is caused not only by the greater number of Ca 2ϩ currents (I Ca ) per unit time (from the increased frequency of action potentials) but also to the greater number of Na ϩ currents (I Na ) per unit time, which raises the intracellular Na ϩ activity and thereby promotes Ca 2ϩ entry and decreases Ca 2ϩ efflux via Na ϩ / Ca 2ϩ exchange Rat cardiac muscle is generally considered to be unusual among mammalian preparations because it usually exhibits a negative force-frequency relationship, i.e., steady-state isometric force is reduced as stimulation frequency is increased Many studies with rat myocardium were conducted at stimulation frequencies, temperatures, and [Ca 2ϩ ] o that bear little resemblance to the physiological condition. Therefore, in the present study we aimed to reexamine the isometric force-frequency relationship in thin rat ventricular trabeculae using a range of frequencies (0.1-12 Hz) encompassing the physiological range, a range of temperatures (24, 30, and 37°C), and a [Ca 2ϩ ] o of 1 mM, which is close to the physiological [Ca 2ϩ ] o of 1.3 mM in rat plasma METHODS Muscle preparation. The experimental procedure and apparatus were similar in most respects to those described in detail previously (23). The strain of rats used was either Wistar (male, ϳ250 g) or LBNf 1 , i.e., first progeny of a cross between female Lewis and male Brown Norway rats (either sex, ϳ250 g). Rats were stunned and then killed by cervical dislocation (Schedule 1 procedure in accordance with United Kingdom Home Office regulations). The hearts were removed and rinsed free of blood with modified Krebs-Henseleit solution, which contained (in mM) 93 NaCl, 20 NaHCO 3 , 1 Na 2 HPO 4 , 1 MgSO 4 , 5 KCl, 1 CaCl 2 , 10 glucose, and 20 Na-acetate with 5 U/l insulin, bubbled with 95% O 2 -5% CO 2 , pH 7.4. Strongly beating, unbranched trabeculae (diameter 80-250 µm) were identified in the right ventricle. The Krebs-Henseleit solution was then exchanged for Krebs to which 25 mM 2,3-butanedione monoxime (BDM) had been added, to minimize irreversible muscle damage during dissection. After 30 min in this solution a trabecula was carefull

Effects of α1- or β-adrenoceptor stimulation on work-loop and isometric contractions of isolated rat cardiac trabeculae

We studied the effects of α1-or β-adrenoceptor stimulation on the contractility of isolated rat ventricular trabeculae at 24 °C using the work-loop technique, which simulates the cyclical changes in length and force that occur during the cardiac cycle. Some muscles were injected with fura-2 to monitor the intracellular Ca2+ transient.Comparison of twitch records revealed that peak force was greater and was reached earlier in work-loop contractions than in corresponding isometric contractions. This was attributed to the changes in muscle length and velocity during work-loop contractions, since the Ca2+ transients were largely unaffected by the length changes.Stimulation of α1-adrenoceptors (with 100 μm phenylephrine) increased net work, power production, the frequency for maximum work, and the frequency for maximum power production (fopt). The increase in net work was due to the positive inotropic effect of phenylephrine, which was similar at all frequencies investigated (0.33–4.5 Hz). The increase in fopt was attributed to an abbreviation of twitch duration induced by α1-stimulation at higher frequencies (> 1 Hz), even though the twitch became longer at 0.33 Hz.β-Adrenoceptor stimulation (with 5 μm isoprenaline) produced marked increases in net work, power output, the frequency for net work, and fopt. These effects were attributed both to the positive inotropic effect of β-stimulation, which was greater at higher frequencies, and to the reduction in twitch duration. β-Stimulation also abolished the frequency-dependent acceleration of twitch duration.The increase in power output and fopt with α1- as well as β-adrenoceptor stimulation suggested that both receptor types may contribute to the effects of catecholamines, released during stress or exercise, although the greater effects of β-stimulation are likely to predominate

Enhanced length-dependent Ca2+ activation in fish cardiomyocytes permits a large operating range of sarcomere lengths

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