TRPC3 cation channel plays an important role in proliferation and differentiation of skeletal muscle myoblasts

During membrane depolarization associated with skeletal excitation-contraction (EC) coupling, dihydropyridine receptor [DHPR, a L-type Ca2+ channel in the transverse (t)-tubule membrane] undergoes conformational changes that are transmitted to ryanodine receptor 1 [RyR1, an internal Ca2+-release channel in the sarcoplasmic reticulum (SR) membrane] causing Ca2+ release from the SR. Canonical-type transient receptor potential cation channel 3 (TRPC3), an extracellular Ca2+-entry channel in the t-tubule and plasma membrane, is required for full-gain of skeletal EC coupling. To examine additional role(s) for TRPC3 in skeletal muscle other than mediation of EC coupling, in the present study, we created a stable myoblast line with reduced TRPC3 expression and without alpha1SDHPR (MDG/TRPC3 KD myoblast) by knock-down of TRPC3 in alpha1SDHPR-null muscular dysgenic (MDG) myoblasts using retrovirus-delivered small interference RNAs in order to eliminate any DHPR-associated EC coupling-related events. Unlike wild-type or alpha1SDHPR-null MDG myoblasts, MDG/TRPC3 KD myoblasts exhibited dramatic changes in cellular morphology (e.g., unusual expansion of both cell volume and the plasma membrane, and multi-nuclei) and failed to differentiate into myotubes possibly due to increased Ca2+ content in the SR. These results suggest that TRPC3 plays an important role in the maintenance of skeletal muscle myoblasts and myotubes.

Función del retículo sarcoplásmico y su papel en las enfermedades cardíacas / Function and role of the sarcoplasmic reticulum in heart disease

The sarcoplasmic reticulum (SR) constitutes the main intracellular calcium store in striated muscle and plays an important role in the regulation of excitation-contraction-coupling (ECC) and of intracellular calcium concentrations during contraction and relaxation. The regulation of ECC occurs due to the interaction among the main proteins of the SR that are the calcium release channel or ryanodine receptor, the Ca2+-ATPase, phospholamban and calsequestrin. Due to the importance of ECC in the physiopathology of a number of cardiac diseases, the role of the SR and its components has been widely investigated in some pathologies, specifically cardiac hypertrophy, heart failure, and hereditary arrhythmias. Therefore, the SR proteins constitute an area of research of great interest for the development of new genetic and pharmacologic therapies; from this derives the importance of understanding the function of the SR. This review analyzes the expression, structure, and function of the main SR proteins, their role on myocardial contraction and relaxation and in the changes that occur in cardiac pathologies.

Ca2+ entry, efflux and release in smooth muscle

Control of smooth muscle is vital for health. The major route to contraction is a rise in intracellular [Ca2+], determined by the entry and efflux of Ca2+ and release and re-uptake into the sarcoplasmic reticulum (SR). We review these processes in myometrium, to better understand excitation-contraction coupling and develop strategies for preventing problematic labours. The main mechanism of elevating [Ca2+] is voltage-gated L-type channels, due to pacemaker activity, which can be modulated by agonists. The rise of [Ca2+] produces Ca-calmodulin and activates MLCK. This phosphorylates myosin and force results. Without Ca2+ entry uterine contraction fails. The Na/Ca exchanger (NCX) and plasma membrane Ca-ATPase (PMCA) remove Ca2+, with contributions of 30 percet and 70 percet respectively. Studies with PMCA-4 knockout mice show that it contributes to reducing [Ca2+] and relaxation. The SR contributes to relaxation by vectorially releasing Ca2+ to the efflux pathways, and thereby increasing their rates. Agonists binding produces IP3 which can release Ca from the SR but inhibition of SR Ca2+ release increases contractions and Ca2+ transients. It is suggested that SR Ca2+ targets K+ channels on the surface membrane and thereby feedback to inhibit excitability and contraction.

Control of dual isoforms of Ca2+ release channels in muscle

Here we compare excitation-contraction coupling in single muscle cells of frogs and rats. Because amphibians have isoform 3 (or b) of the ryanodine receptor/Ca2+ release channel, in addition to 1 (a), which is also present in the mammal, any extra feature present in the frog may in principle be attributed to isoform 3. Ca2+ release under voltage clamp depolarization has a peak and a steady phase in both taxonomic classes, but the peak is more marked in the frog, where the ratio of amplitudes of the two phases is voltage-dependent. This dependence is a hallmark of CICR. Confocal imaging identified Ca2+ sparks in the frog, but not in the voltage-clamped rat cells. Because Ca2+ sparks involve CICR both observations indicate that the contribution of CICR is minor or null in the mammal. The "couplon" model well accounts for observations in the frog, but assumes a structure that we now know to be valid only for the rat. A revised model is proposed, whereby the isoform 3 channels, located parajunctionally, are activated by CICR and contribute its characteristic global and local features. Several issues regarding the roles of different channels remain open to further study.

Influence of the sarcoplasmic reticulum on the inotropic responses of the rat myocardium resulting from changes in rate and rhythm

1. The role of the sarcoplasmic reticulum (SR) in the inotropic responses produced by changes in stimulation rate and rhythm and resting tension was investigated in the rat myocardium. 2. Rat papillary muscles contracting isometrically (basic stimulation rate = 30/min) were superfused in vitro with normal Krebs solution and after addition of ryanodine (1 microM). Post-rest potentiation was obtained after pauses of 5, 10, 15, 30, 60 and 120 s, and the stimulation rate was changed from 6 to 90 bpm. Post-extrasystolic potentiation was induced by interpolating an extra stimulus after an interval of 413 +/- 15 ms. NiCl2 (2 mM) was used to confirm that contractions obtained after SR blockade with ryanodine were activated only by sarcolemmal calcium influx. 3. In the presence of ryanodine, the post-rest potentiation phenomenon disappears and the force-frequency relationship changes from the typical force decrease produced by rate increase to force increase. Under the effect of ryanodine, resting tension increased with the increase in stimulation rate. This behavior was enhanced by reducing extracellular KCl from 5.4 mM to 1 mM. This maneuver decreases Na(+)-K(+)-ATPase and increases intracellular Na+ activity, which reduces Ca2+ extrusion through the Na(+)-Ca2+ exchange mechanism. 4. SR participation in the post-extrasystolic potentiation phenomenon is also suggested because ryanodine treatment reversed the extrasystolic force depression into potentiation. In the presence of ryanodine, blockade of Ca2+ influx with NiCl2 (2 mM) abolished isometric contractions indicating that after SR blockade contractions are mainly dependent on sarcolemmal Ca2+ influx. 5. The results suggest that the SR is involved in the genesis of post-rest potentiation and contributes to the typical force-frequency relationship of the rat myocardium and to the post-extrasystolic potentiation phenomenon. Moreover, SR activity seems to be important for the maintenance of low resting tension in the cardiac muscle, which may represent a safety factor against contractures during inotropic changes produced in rate and rhythm.

The effect of verapamil and Ca free solution on mechanical and electrical properties in fast twitch mammalian skeletal muscle

Fueron estudiados los efectos del verapamil (0.01-0.1 mM) y de la solución libre de Ca (0 Ca, 3 mM MgCl2, 5 mM EGTA) sobre las propiedades eléctricas y mecánicas del músculo extensor digitorum longus de rata. La solución libre de Ca redujo las tensiones de la contracción simple y tetánica. El verapamil no afectó la respuesta a pulsos simples, pero a medida que la frecuencia de estimulación se incrementó, la tensión pico se redujo gradualmente: efecto frecuencia-dependiente. Ambas soluciones disminuyeron significativamente la tensión de la contractura por cafeína (30 mM). Se observó que las fibras en solución libre de Ca se despolarizó (6 mV) y las +dv/dt. -dv/dt y el "overshoot" del potencial de acción tambén fueron reducidos. En cambio, el verapamil no afectó el potencial de reposo. Las fibras expuestas a 0.1 mM de verapamil perdieron la capacidad de responder a altas frecuencias de estimulación. Concentraciones inferiores de verapamil (0.01 mM) no afectaron la respuesta repetitiva, pero la tensión tetánica disminuyó. Estos hallazgos sostienen parcialmente la hipótesis acerca de la dependencia del músculo esquelético de mamífero del Ca extracelular. El verapamil tendría varios mecanismos de acción: a) bloquente de los canales de Ca; b) efecto anestésico local; c) inhibidor de la liberación de Ca desde el retículo sarcoplásmico

Efecto inhibitorio del propranolol sobre la relajacion miocardica no mediado a traves del bloqueo beta / Inhibitory effect of propranolol on myocardial relaxation not mediated by beta blockade

En corazones aislados, con frecuencia cardíaca y flujo coronario constants, se estudiaron los efectos del d- y d, l-propranolol sobre la relajación miocárcica. La infusión del d, l-propranolol 10**-5M produjo una disminución significativa en la máxima velocidad de conhtracción (+T) de 48 ñ 1.5 % (P < 0.05) y una disminución proporcionalmente mayor de la máxima velocidad de relajación (-T) de 58 ñ 2 % con un aumento significativo de la relación entre ambas velocidades (+T/-T) de 28.4 ñ 6.2 % (1.35 ñ 0.03 a 1.74 ñ 0.09). La constante de tiempo de la relajación en su porción exponencial (Tau) se prolongó significativamente en 64 ñ 17.5 % (de 3.8 ñ l.4 a 61 ñ 6 ms) (P < 0.05). Una reducción en +T de 52.2 ñ 4.1% provocada por perfusión con baja concentración de calcio (0.5 mM) también aumentó +T/-T y Tau en 11 ñ 4.5 % y 27 ñ 6 % respectivamente (P < 0.05). Estos aumentos fueron, sin embargo, menores que los provocados por d- y d, 1- propranolol (P < 0.05). El efecto de la concentración baja de calcio sobre +T, +T/-T y Tau fue completamente reversible, en tanto que la reversión del efecto inotrópico negativo de d- o d, l-propranolol por calcio no revirtió dla disminución en -T ni los aumentos de +T/-T y Tau provocados por la droga. Resultados similares se obtuvieron en corazones deplecionados de catecolominas. El d/propranolol no afectó la sensibilidad al calcio de trabéculas ventriculares por derechas de gato peladas por procedimientos químicos. La captura de calcio por el sistema retículo...