Surface pH and the control of intracellular pH in cardiac and skeletal muscle

Both surface pH (pHs) and intracellular pH (pHi) were measured using single- and double-barreled pH-sensitive microelectrodes in isolated sheep cardiac Purkinje strands, rabbit and cat papillary muscle, and mouse and rat soleus muscle. Superfusion of the preparations with a relatively low buffered solution (containing 5 mM HEPES buffered to control pH) causes surface acidosis that correlates with efflux of metabolically produced acids in the unstirred layer of fluid surrounding the tissue. Acidification of the surface layer induces a slower acid change of pHi and depresses the rate of proton extrusion following an imposed intracellular acid load. In cardiac preparations, the lowering of pHi correlates with depression of twitch tension. Transient changes of pHs and pHi are seen when a weak acid or base is suddenly added to, or removed from the superfusion solution. Indirect evidence of the presence of carbonic anhydrase in the extracellular surface layer is obtained from analysis of transient pHs changes in presence and absence of acetazolamide.

Low Ba-induced pacemaker current in well-polarized cat papillary muscle

It has previously been reported that superfusion of normally quiescent mammalian ventricular muscle with low concentrations of Ba (less than 0.3 mM) can induce spontaneous activity with maximum diastolic potentials (MDP) that are similar to the normal resting potential (-80 mV or larger). The mechanism for this activity was studied in cat papillary muscle sucrose-gap preparations under current clamp and voltage-clamp conditions. Hyperpolarizing current pulses decreased or abolished the amplitude of the pacemaker potential in a voltage-dependent manner. When Ba concentration was increased to 2 mM the MDP depolarized by approximately 20 mV. Hyperpolarizing steps under these conditions abolished the diastolic depolarization, also in a voltage-dependent manner. Voltage clamping the preparation at the MDP during superfusion of 0.2 mM Ba revealed a time-dependent, inwardly directed current. Hyperpolarizing voltage-clamp steps from a holding potential of -50 mV showed that this current was maximal at approximately -70 mV and frequently reversed at membrane potentials of approximately -95 to -115 mV. The time course of this current was biexponential, and the time constant of the faster component decreased with larger hyperpolarization. When the same voltage-clamp protocol was repeated in the presence of 2 mM Ba, no time-dependent current change was detected. In four out of five experiments, Cs (2.5 mM) reduced (but never abolished) the amplitude of the low Ba-induced current. Our results do not support the hypothesis that a hyperpolarization-induced current (iF-like current) is responsible for the automaticity in well-polarized ventricular muscle at low Ba concentrations. Instead, our data suggest that this pacemaker activity is the result of a Ba-induced, time-dependent blockade of the inward rectifier potassium current (iK1).

Improvement in relaxation by nifedipine in hypoxic isometric cat papillary muscle

Hypoxia has been demonstrated to cause impairment of myocardial relaxation. This impairment of relaxation is particularly pronounced during early reoxygenation. This study was undertaken in 24 isometric cat papillary muscles at 38 degrees C to determine if nifedipine can influence the impairment of relaxation produced during reoxygenation following hypoxia. A dose of nifedipine was chosen that produced only a minimal depression of peak systolic tension and no change in the half time to relaxation (RT 1/2) under well-oxygenated conditions. Thirty minutes of hypoxia were produced in 12 muscles, and systolic tension decreased by the same amount in muscles treated or not treated with nifedipine. During early hypoxia in the absence of nifedipine, RT1/2 was significantly prolonged (P less than 0.01) from 104 +/- 7 to 126 +/- 9 ms. After pretreatment with nifedipine, the change in RT1/2 with hypoxia was not significant. More striking was the near abolition of the marked impairment of relaxation seen during early reoxygenation (238 +/- 33 ms without nifedipine and 128 +/- 8 ms with nifedipine, P less than 0.01). These data establish that, although nifedipine only minimally attenuates the relaxation impairment early during hypoxia, this agent can substantially reduce the impairment of relaxation produced by early reoxygenation.

Contribution of activation-inactivation dynamics to the impairment of relaxation in hypoxic cat papillary muscle

Previous investigation of conventional isometric twitches of normothermic cat papillary muscle has shown that hypoxia prolongs relaxation, and this prolongation is actually accentuated during early reoxygenation. Our aim was to identify how hypoxia and reoxygenation affect the coupled processes of activation and inactivation that govern the time course of internally generated contractile tension (Ti). Activation and inactivation are modeled as first-order processes with rate constants ka and ki, respectively, and the overall isometric muscle as an underdamped second-order lag system driven by Ti. The analytical expression (To) for the externally recorded tension is dominated by two exponential terms incorporating ka and ki. Accurate least-squares fits of digitized twitches to To yielded estimates of ka and ki at 1- to 3-min intervals during control oxygenation, hypoxia, and early and late reoxygenation. Results follow. Compared with control, normothermic hypoxia prolonged activation [at 15 min ka decreased 61% from control, 35.5 +/- 6 (SE) s-1, P less than 0.05] and accelerated inactivation (at 15 min, ki increased 69% from control, 6.0 +/- 0.5 s-1, P less than 0.05). In early reoxygenation (1-3 min) activation remained impaired and inactivation returned to control levels (ki decreased 16% from control, NS). In late reoxygenation (15 min) both processes reverted to control. Thus inactivation kinetics can be dissociated from activation kinetics. Impaired relaxation in normothermic hypoxia is due to prolonged activation, whereas inactivation is actually accelerated. The further impairment of relaxation in early reoxygenation is due to rapid return of inactivation to control at a time when activation is still prolonged.