Relaxation of rabbit ventricular muscle by Na-Ca exchange and sarcoplasmic reticulum calcium pump. Ryanodine and voltage sensitivity.

Abstract

We studied relaxation during rapid rewanning of rabbit ventricular muscles that had been activated by rapid cooling. Rewanning from 1° to 30° C (in <0.5 second) activates mechanisms that contribute to the reduction of intracellular calcium concentration and thus relaxation (e.g., sarcoplasmic reticulum [SR] calcium pump and sarcolemmal Na-Ca exchange and calcium pump). Rapid rewanning in normal Tyrode's solution induces relaxation with a half-time (t^) of 217±14 msec (mean±SEM). During cold exposure, changing the superfusate to a sodiumfree, calcium-free medium with 2 mM CoCl 2 (to eliminate Na-Ca exchange) slightly slows relaxation upon rewanning in the same medium (t w =279±44 msec). Addition of 10 mM caffeine (which prevents SR calcium sequestration) to normal Tyrode's solution during cold superfusion slows relaxation somewhat more (t 1/2 =376±31 msec) than sodium-free, calciumfree solution. However, if both interventions are combined (sodium-free+caffeine) during the cold exposure and rewarming, the relaxation is greatly slowed (t 1/1 =2,580±810 msec). These results suggest that either the SR calcium pump or, to a lesser extent, sarcolemmal Na-Ca exchange can produce rapid relaxation, but if both systems are blocked, relaxation is very slow. If muscles are equilibrated with 500 nM ryanodine before cooling, relaxation upon rewarming is not greatly slowed (t 1/2 =266±37 msec) even if sodium-free, calcium-free solution is applied during the cold and rewarming phases (t l/2 =305±66 msec). This result suggests that ryanodine does not prevent the SR from accumulating calcium to induce relaxation. Relaxation in the presence of 10 mM caffeine appears to depend on a simple 3 : 1 Na-Ca exchange since relaxation is slowed by extracellular sodium reduction but stays constant with simultaneous reduction of extracellular sodium concentration and extracellular calcium concentration (where [Na] Received October 12, 1988; accepted January 13, 1989. occur, calcium must be removed from the myofilaments. At least three mechanisms are thought to be involved in reducing myoplasmic calcium: 1) the ATP-dependent SR calcium pump, 2) the sarcolemmal Na-Ca exchange, and 3) the sarcolemmal CaATPase pump. It is also possible that mitochondria contribute to relaxation by sequestering calcium 3 and other intracellular constituents that have very slow calcium-binding kinetics (relative to troponin C) could also be involved in relaxation. It is generally assumed that the SR calcium pump is the prtaeipaJ aecfeaeisfla responsible for relaxation. However, it is becoming increasingly dear that Na-Ca exchange can transport sufficient calcium rapidly enough to be involved in contraction and relaxation. 47 Here we investigate the relative contributions of the SR calcium pump and Na-Ca exchange in cardiac muscle relaxation.

Characterization of the 70 kDa polypeptide of the Na/Ca exchanger

“…Na\Ca exchange is of major importance for Ca# + homeostasis in myocardial cells [1][2][3]. Many molecular and kinetic aspects of the exchanger have been characterized [4,5], and successful solubilization and reconstitution of the exchanger have been performed from different tissues [6][7][8][9][10][11][12][13][14].…”

Processes that remove calcium from the cytoplasm during excitation‐contraction coupling in intact rat heart cells.

“…After the peak of [Ca2"I is reached, [Ca2+I (or,in some studies, fluorescence ratio) then declines exponentially with a time constant of 200 ms or somewhat less (at 21°C) (Bers, Lederer & Berlin, 1990;O'Neill, Valdeolmillos, Lamont, Donoso & Eisner, 1991). This declining phase of the cytoplasmic [Ca2"] transient is thought to result mainly from sequestration of Ca2" by the SR (Bers & Bridge, 1989) as the time constant of the exponential decline is increased several-fold by caffeine (O'Neill et al 1991) and by specific SR Ca2`-ATPase inhibitors, such as thapsigargin (Kirby, Sagara, Gaa, Inesi, Lederer & Rogers, 1992) and cyclopiazonic acid (Balke & Wier, 1992). Consistent with this, it has also been shown that under most circumstances, extrusion of Ca2+ from the cell by sodium-calcium (Na+-Ca2+) exchange contributes relatively little to the decline of [Ca2+]i (Bers et al 1990;O'Neill et al 1991) and that yet lesser contributions are made by the Ca2+-pumping ATPase of the surface membrane (SL) and the Ca2" 'uniporter' of the mitochondria.…”

Chronic Administration of KB-R7943 Induces Up-regulation of Cardiac NCX1

“…SR Ca 2ϩ -ATPase recycles Ca 2ϩ from the cytosol into the lumen of the SR, and NCX1 mediates the movement of [Ca 2ϩ ] i across the sarcolemma to the extracellular space. NCX1 transports ϳ28% of the cytosolic Ca 2ϩ during a contractionrelaxation cycle in large animals and humans, with 70% being reaccumulated in the SR (via SR Ca 2ϩ -ATPase) (2)(3)(4). Alterations in any of the activities associated with this complex process causes a corresponding change in the amount of Ca 2ϩ released from the SR, and the resulting force of cardiac contraction.…”