Intracellular Mg2+ diffusion within isolated rat skeletal muscle fibers

Whole-cell voltage clamp on skeletal muscle fibers with the silicone-clamp technique

Control of membrane voltage and membrane current measurements are of critical importance for the study of numerous aspects of skeletal muscle physiology and pathophysiology. The silicone-clamp technique makes use of a conventional patch-clamp apparatus to achieve whole-cell voltage clamp of a restricted portion of a fully differentiated adult skeletal muscle fiber. The major part of an isolated muscle fiber is insulated from the extracellular medium with silicone grease and the tip of a single microelectrode connected to the amplifier is then inserted within the fiber through the silicone layer. The method is extremely easy to implement. It represents an alternative to the traditional vaseline-gap isolation and two or three microelectrodes voltage-clamp techniques. The present chapter reviews the benefits of the silicone-clamp technique and provides updated detailed insights into its practical implementation.

Molecules in motion: influences of diffusion on metabolic structure and function in skeletal muscle

Metabolic processes are often represented as a group of metabolites that interact through enzymatic reactions, thus forming a network of linked biochemical pathways. Implicit in this view is that diffusion of metabolites to and from enzymes is very fast compared with reaction rates, and metabolic fluxes are therefore almost exclusively dictated by catalytic properties. However, diffusion may exert greater control over the rates of reactions through: (1) an increase in reaction rates; (2) an increase in diffusion distances; or (3) a decrease in the relevant diffusion coefficients. It is therefore not surprising that skeletal muscle fibers have long been the focus of reaction-diffusion analyses because they have high and variable rates of ATP turnover, long diffusion distances, and hindered metabolite diffusion due to an abundance of intracellular barriers. Examination of the diversity of skeletal muscle fiber designs found in animals provides insights into the role that diffusion plays in governing both rates of metabolic fluxes and cellular organization. Experimental measurements of metabolic fluxes, diffusion distances and diffusion coefficients, coupled with reaction-diffusion mathematical models in a range of muscle types has started to reveal some general principles guiding muscle structure and metabolic function. Foremost among these is that metabolic processes in muscles do, in fact, appear to be largely reaction controlled and are not greatly limited by diffusion. However, the influence of diffusion is apparent in patterns of fiber growth and metabolic organization that appear to result from selective pressure to maintain reaction control of metabolism in muscle.

New insight into the functioning of nitric oxide-receptive guanylyl cyclase: physiological and pharmacological implications

The cellular counterpart of the "soluble" guanylyl cyclase found in tissue homogenates over 30 years ago is now recognized as the physiological receptor for nitric oxide (NO). The ligand-binding site is a prosthetic haem group that, when occupied by NO, induces a conformational change in the protein that propagates to the catalytic site, triggering conversion of GTP into cGMP. This review focuses on recent research that takes this basic information forward to the beginnings of a quantitative depiction of NO signal transduction, analogous to that achieved for other major transmitters. At its foundation is an explicit enzyme-linked receptor mechanism for NO-activated guanylyl cyclase that replicates all its main properties. In cells, NO signal transduction is subject to additional, activity-dependent modifications, notably through receptor desensitization and changes in the activity of cGMP-hydrolyzing phosphodiesterases. The measurement of these parameters under varying conditions in rat platelets has made it possible to formulate a cellular model of NO-cGMP signaling. The model helps explain cellular responses to NO and their modification by therapeutic agents acting on the guanylyl cyclase or phosphodiesterase limbs of the pathway.

Quantification of Mg2+ extrusion and cytosolic Mg2+-buffering in Xenopus oocytes

Intracellular Mg(2+) buffering and Mg(2+) extrusion were investigated in Xenopus laevis oocytes. Mg(2+) or EDTA were pressure injected and the resulting changes in the intracellular Mg(2+) concentration were measured simultaneously with Mg(2+)-selective microelectrodes. In the presence of extracellular Na(+), injected Mg(2+) was extruded from the oocytes with an estimated v(max) and K(M) of 74 pmol cm(-2)s(-1) and 1.28 mM, respectively. To investigate genuine cytosolic Mg(2+) buffering, measurements were carried out in the nominal absence of extracellular Na(+) to block Mg(2+) extrusion, and during the application of CCCP (inhibiting mitochondrial uptake). Under these conditions, Mg(2+) buffering calculated after both MgCl(2) and EDTA injections could be described by a buffer equivalent with a concentration of 9.8mM and an apparent dissociation constant, K(d-app), of 0.6mM together with an [ATP](i) of 0.9 mM with a K(d-app) 0.12 mM. Xenopus oocytes thus possess highly efficient mechanisms to maintain their intracellular Mg(2+) concentration.

Whole-cell voltage clamp on skeletal muscle fibers with the silicone-clamp technique

Control of membrane voltage and membrane current measurements are of strong interest for the study of numerous aspects of skeletal muscle physiology and pathophysiology. The silicone-clamp technique makes use of a conventional patch-clamp apparatus to achieve whole-cell voltage clamp of a restricted portion of a fully differentiated adult skeletal muscle fiber. The major part of an isolated muscle fiber is insulated from the extracellular medium with silicone grease, and the tip of a single microelectrode connected to the amplifier is then inserted within the fiber through the silicone layer. This method represents an alternative to the traditional vaseline-gap isolation and two or three microelectrode voltage-clamp techniques. This chapter reviews the main benefits of the silicone-clamp technique and provides detailed insights into its practical implementation.

Spatial and temporal Ca2+, Mg2+, and ATP2- dynamics in cardiac dyads during calcium release

We have constructed a three-dimensional reaction-diffusion model of the mammalian cardiac calcium release unit. We analyzed effects of diffusion coefficients, single channel current amplitude, density of RyR channels, and reaction kinetics of ATP(2-) with Ca(2+) and Mg(2+) ions on spatiotemporal concentration profiles of Ca(2+), Mg(2+), and ATP(2-) in the dyadic cleft during Ca(2+) release. The model revealed that Ca(2+) concentration gradients persist near RyRs in the steady state. Even with low number of open RyRs, peak [Ca(2+)] in the dyadic space reached values similar to estimates of luminal [Ca(2+)] in approximately 1 ms, suggesting that during calcium release the Ca(2+) gradient moves from the cisternal membrane towards the boundary of the dyadic space with the cytosol. The released Ca(2+) bound to ATP(2-), and thus substantially decreased ATP(2-) concentration in the dyadic space. The released Ca(2+) could also replace Mg(2+) in its complex with ATP(2-) during first milliseconds of release if dissociation of MgATP was fast. The results suggest that concentration changes of Ca(2+), Mg(2+), and ATP(2-) might be large and fast enough to reduce dyadic RyR activity. Thus, under physiological conditions, termination of calcium release may be facilitated by the synergic effect of the construction and chemistry of mammalian cardiac dyads.

Inhibition of cAMP-dependent protein kinase under conditions occurring in the cardiac dyad during a Ca2+ transient

The space between the t-tubule invagination and the sarcoplasmic reticulum (SR) membrane, the dyad, in ventricular myocytes has been predicted to experience very high [Ca2+] for short periods of time during a Ca2+ transient. The dyadic space accommodates many protein kinases responsible for the regulation of Ca2+ handling proteins of the cell. We show in vitro that cAMP-dependent protein kinase (PKA) is inhibited by high [Ca2+] through a shift in the ratio of CaATP/MgATP toward CaATP. We further generate a three-dimensional mathematical model of Ca2+ and ATP diffusion within dyad. We use this model to predict the extent to which PKA would be inhibited by an increased CaATP/MgATP ratio during a Ca2+ transient in the dyad in vivo. Our results suggest that under normal physiological conditions a myocyte paced at 1 Hz would experience up to 55% inhibition of PKA within the cardiac dyad, with inhibition averaging 5% throughout the transient, an effect which becomes more pronounced as the myocyte contractile frequency increases (at 7 Hz, PKA inhibition averages 28% across the dyad throughout the duration of a Ca2+ transient).