Length-dependent activation by Ba2+ and Sr2+ of skinned cardiac and skeletal muscle of the rabbit

Over a wide range of sarcomere lengths, force activation by Ca2+, Ba2+, and Sr2+ was studied in papillary muscle and in fast skeletal fibers of the gracilis muscle of the rabbit, both skinned by means of freeze drying. The length-tension relations of Ba2+ activation differ significantly from those of Sr2+ and Ca2+ activation with respect to both the value and the position of the maximum. At (almost) full activation, force induced in gracilis muscle by Ba2+ was 50% of the developed force induced by Ca2+. The position of the Sr2+ sensitivity curve for papillary muscle preparations is independent of sarcomere length, in contrast to the position of the Ca2+ sensitivity curves. The binding of Sr2+ to the papillary preparation proves to be very stable as observed from the long-lasting relaxation after activation. Immersion of the papillary preparation in the relaxation fluid after activation with Ba2+ results in a tension transient: a rise in tension followed by a decrease was observed. The maximal value of the tension transient was up to twice the steady tension, dependent on Ba2+ concentration. The steady-state tension was approximately 50% of the Ca2(+)-induced tension. Ba2+ sensitivity curves are not sigmoidal but show a maximum. Above [Ba2+] greater than 10(-5) to 10(-4) M (dependent on sarcomere length) tension decreased. These observations suggest that two counteracting processes govern Ba2+ contraction in papillary muscle preparations, namely activation and inhibition.

Mechanical transients of single toad stomach smooth muscle cells. Effects of lowering temperature and extracellular calcium.

Smooth muscle's slow, economical contractions may relate to the kinetics of the crossbridge cycle. We characterized the crossbridge cycle in smooth muscle by studying tension recovery in response to a small, rapid length change (i.e., tension transients) in single smooth muscle cells from the toad stomach (Bufo marinus). To confirm that these tension transients reflect crossbridge kinetics, we examined the effect of lowering cell temperature on the tension transient time course. Once this was confirmed, cells were exposed to low extracellular calcium [( Ca2+]o) to determine whether modulation of the cell's shortening velocity by changes in [Ca2+]o reflected the calcium sensitivity of one or more steps in the crossbridge cycle. Single smooth muscle cells were tied between an ultrasensitive force transducer and length displacement device after equilibration in temperature-controlled physiological saline having either a low (0.18 mM) or normal (1.8 mM) calcium concentration. At the peak of isometric force, after electrical stimulation, small, rapid (less than or equal to 1.8% cell length in 3.6 ms) step stretches and releases were imposed. At room temperature (20 degrees C) in normal [Ca2+]o, tension recovery after the length step was described by the sum of two exponentials with rates of 40-90 s-1 for the fast phase and 2-4 s-1 for the slow phase. In normal [Ca2+]o but at low temperature (10 degrees C), the fast tension recovery phase slowed (apparent Q10 = 1.9) for both stretches and releases whereas the slow tension recovery phase for a release was only moderately affected (apparent Q10 = 1.4) while unaffected for a stretch. Dynamic stiffness was determined throughout the time course of the tension transient to help correlate the tension transient phases with specific step(s) in the crossbridge cycle. The dissociation of tension and stiffness, during the fast tension recovery phase after a release, was interpreted as evidence that this recovery phase resulted from both the transition of crossbridges from a low- to high-force producing state as well as a transient detachment of crossbridges. From the temperature studies and dynamic stiffness measurements, the slow tension recovery phase most likely reflects the overall rate of crossbridge cycling. From the tension transient studies, it appears that crossbridges cycle slower and have a longer duty cycle in smooth muscle. In low [Ca2+]o at 20 degrees C, little effect was observed on the form or time course of the tension transients.(ABSTRACT TRUNCATED AT 400 WORDS)

Peeled mammalian skeletal muscle fibers. Possible stimulation of Ca2+ release via a transverse tubule-sarcoplasmic reticulum mechanism.

Single muscle fibers from rabbit soleus and adductor magnus and from semitendinosus muscles were peeled to remove the sarcolemma and then stimulated to release Ca2+ by (a) caffeine application or (b) ionic depolarization accomplished via substitution of choline chloride for potassium propionate at constant [K+] X [Cl-] in the bathing solution. Each stimulus, ionic or caffeine, elicited an isometric tension transient that appeared to be due to Ca2+ released from the sarcoplasmic reticulum (SR). The peak magnitude of the ionic (Cl- -induced) tension transient increased with increasing Cl- concentration. The application of ouabain to fibers after peeling had no effect on either type of tension transient. However, soaking the fibers in a ouabain solution before peeling blocked the Cl- -induced but not the caffeine-induced tension transient, which suggests that ouabain's site of action is extracellular, perhaps inside transverse tubules (TTs). Treating the peeled fibers with saponin, which should disrupt TTs to a greater extent than SR membrane, greatly reduced or eliminated the Cl- -induced tension transient without significantly altering the caffeine-induced tension transient. These results suggest that the Cl- -induced tension transient is elicited via stimulation of sealed, polarized TTs rather than via ionic depolarization of the SR.

Simulated calcium current can both cause calcium loading in and trigger calcium release from the sarcoplasmic reticulum of a skinned canine cardiac Purkinje cell.

Skinned canine cardiac Purkinje cells were stimulated by regularly repeated microinjection-aspiration sequences that were programmed to simulate the fast initial component of the transsarcolemmal Ca2+ current and the subsequent slow component corresponding to noninactivating Ca2+ channels. The simulated fast component triggered a tension transient through Ca2+-induced release of Ca2+ from the sarcoplasmic reticulum (SR). The simulated slow component did not affect the tension transient during which it was first introduced but it potentiated the subsequent transients. The potentiation was not observed when the SR function had been destroyed by detergent. The potentiation decreased progressively when the slow component was separated by an increasing time interval from the fast component. The potentiation was progressive over several beats under conditions that decreased the rate of Ca2+ accumulation into the SR (deletion of calmodulin from the solutions; a decrease of the temperature from 22 to 12 degrees C). In the presence of a slow component, an increase of frequency caused a positive staircase, and the introduction of an extrasystole caused a postextrasystolic potentiation. There was a negative staircase and no postextrasystolic potentiation in the absence of a slow component. These results can be explained by a time- and Ca2+-dependent functional separation of the release and accumulation processes of the SR, rather than by Ca2+ circulation between anatomically distinct loading and release compartments. The fast initial component of transsarcolemmal Ca2+ current would trigger Ca2+ release, whereas the slow component would load the SR with an amount of Ca2+ available for release during the subsequent tension transients.