Effects of diltiazem or verapamil on calcium uptake and release from chicken skeletal muscle sarcoplasmic reticulum

SERCA-1 conformational change exerted by the Ca2+-channel blocker diltiazem affects mammalian skeletal muscle function

In skeletal muscle (SM), inward Ca2+-currents have no apparent role in excitation-contraction coupling (e-c coupling), however the Ca2+-channel blocker can affect twitch and tetanic muscle in mammalian SM. Experiments were conducted to study how diltiazem (DLZ) facilitates e-c coupling and inhibits contraction. 1) In complete Extensor Digitorum Longus (EDL) muscle and single intact fibres, 0.03 mM DLZ causes twitch potentiation and decreases force during tetanic activity, with increased fatigue. 2) In split open fibres isolated from EDL fibres, DLZ inhibits sarcoplasmic reticulum (SR) Ca2+-loading in a dose-dependent manner and has a potentiating effect on caffeine-induced SR Ca2+-release. 3) In isolated light SR (LSR) vesicles, SERCA1 hydrolytic activity is not affected by DLZ up to 0.2 mM. However, ATP-dependent Ca2+-uptake was inhibited in a dose-dependent manner at a concentration where e-c coupling is changed. 4) The passive Ca2+-efflux from LSR was reduced by half with 0.03 mM diltiazem, indicating that SR leaking does not account for the decreased Ca2+-uptake. 5) The denaturation profile of the SERCA Ca2+-binding domain has lower thermal stability in the presence of DLZ in a concentration-dependent manner, having no effect on the nucleotide-binding domain. We conclude that the effect of DLZ on SM is exerted by crossing the sarcolemma and interacting directly with the SERCA Ca2+-binding domain, affecting SR Ca2+-loading during relaxation, which has a consequence on SM contractility. Diltiazem effect on SM could be utilized as a tool to understand SM e-c coupling and muscle fatigue.

Phosphatidylinositol 3,5-bisphosphate regulates Ca2+ transport during yeast vacuolar fusion through the Ca2+ ATPase Pmc1

The transport of Ca²⁺ across membranes precedes the fusion and fission of various lipid bilayers. Yeast vacuoles under hyperosmotic stress become fragmented through fission events that requires the release of Ca²⁺ stores through the TRP channel Yvc1. This requires the phosphorylation of phosphatidylinositol-3-phosphate (PI3P) by the PI3P-5-kinase Fab1 to produce transient PI(3,5)P₂ pools. Ca²⁺ is also released during vacuole fusion upon trans-SNARE complex assembly, however, its role remains unclear. The effect of PI(3,5)P₂ on Ca²⁺ flux during fusion was independent of Yvc1. Here, we show that while low levels of PI(3,5)P₂ were required for Ca²⁺ uptake into the vacuole, increased concentrations abolished Ca²⁺ efflux. This was as shown by the addition of exogenous dioctanoyl PI(3,5)P₂ or increased endogenous production of by the hyperactive fab1^T2250A mutant. In contrast, the lack of PI(3,5)P₂ on vacuoles from the kinase dead fab1^EEE mutant showed delayed and decreased Ca²⁺ uptake. The effects of PI(3,5)P₂ were linked to the Ca²⁺ pump Pmc1, as its deletion rendered vacuoles resistant to the effects of excess PI(3,5)P₂ . Experiments with Verapamil inhibited Ca²⁺ uptake when added at the start of the assay, while adding it after Ca²⁺ had been taken up resulted in the rapid expulsion of Ca²⁺ . Vacuoles lacking both Pmc1 and the H⁺ /Ca²⁺ exchanger Vcx1 lacked the ability to take up Ca²⁺ and instead expelled it upon the addition of ATP. Together these data suggest that a balance of efflux and uptake compete during the fusion pathway and that the levels of PI(3,5)P₂ can modulate which path predominates.

Effects of verapamil and gadolinium on caffeine-induced contractures and calcium fluxes in frog slow skeletal muscle fibers

In this work, we tested whether L-type Ca(2+ )channels are involved in the increase of caffeine-evoked tension in frog slow muscle fibers. Simultaneous net Ca(2+) fluxes and changes in muscle tension were measured in the presence of caffeine. Isometric tension was recorded by a mechanoelectrical transducer, and net fluxes of Ca(2+) were measured noninvasively using ion-selective vibrating microelectrodes. We show that the timing of changes in net fluxes and muscle tension coincided, suggesting interdependence of the two processes. The effects of Ca(2+)channel blockers (verapamil and gadolinium) were explored using 6 mM: caffeine; both significantly reduced the action of caffeine on tension and on calcium fluxes. Both caffeine-evoked Ca(2+) leak and muscle tension were reduced by 75% in the presence of 100 microM: GdCl(3), which also caused a 92% inhibition of net Ca(2+) fluxes in the steady-state condition. Application of 10 microM: verapamil to the bath led to 30% and 52% reductions in the Ca(2+)leak caused by the presence of caffeine for the peak and steady-state values of net Ca(2+) fluxes, respectively. Verapamil (10 microM): caused a 30% reduction in the maximum values of caffeine-evoked muscle tension. Gd(3+)was a more potent inhibitor than verapamil. In conclusion, L-type Ca(2+) channels appear to play the initial role of trigger in the rather complex mechanism of slow fiber contraction, the latter process being mediated by both positive Ca(2+)-induced Ca(2+ )release and negative (Ca(2+) removal from cytosol) feedback loops.