Calcium tolerant ventricular myocytes prepared by preincubation in a "KB medium".

Abstract

1. Ventricular myocytes were isolated by means of collagenase and hyaluronidase. Hearts of rats or guinea pigs were perfused in a retrograde manner; bovine ventricular tissue was incubated in from of tissue chunks. 2. In “low Ca medium” (aCa=1 μM), the myocytes beat spontaneously. Resting potentials of −4 mV and input resistances of 1 MΩ, associated with a fast decay of aKi, indicated “hyperpermeability” of the cell membrane. Within 2 h, the cells aged: they granulated (swelling of mitochondria), contracted, rounded up and finally disaggregated. When the superfusate was changed to Tyrode solution (3.6 mM CaCl2), 90–98% of those cells responded with the “Ca paradox”. Initially, aCai increased from 0.2 μM to >10 μM, and the sarcomere length (SL) shortened to <1.5 μm. Within the following 3 min aCai renormalized but the contracture was sustained. After another 5 min, the SL shortened to <1.2 μm, but a delay of 1–2 min passed before aCai started to increase towards aCa0. The 2–10% Ca tolerant cells beat spontaneously. Their resting potentials were between −10 and −70 mV, their input resistances between 4 and 20 MΩ. Repolarization by anodal current flow could restore the sodium spike but not the plateau of the action potential. 3. Freshly prepared myocytes were pre-incubated in a KB medium composed of 85 mM KCl, 30 mM K2HPO4, 5 mM MgSO4, 5 mM Na2ATP, 5 mM pyruvate, 5 mM succinate, 5 mM β-OH-butyrate, 5 mM creatine, adjusted with KOH to pH 7.2 and with EGTA to pCa 7.5. The minimum time of preincubation was 1 h, the maximum 5 days when the cells were stored at 5°C. The conditioning with the KB medium improved both Ca tolerance and electrophysiology significantly. About 70% of the rod shaped myocytes were Ca tolerant. These cells had resting potentials between −75 and −90 mV, the membrane resistances were around 7 kΩ×cm2. The action potentials showed well pronounced plateaus and lasted for 125 ms (rat), 300 ms (guinea pig) or 400 ms (bovine). Their shape showed the typical species related pecularities. 4. We discuss that both ageing and Ca intolerance of the un-conditioned cells result from Ca overload through the hyperpermeable membrane, and that Ca overload causes ATP depletion via mitochondrial disfunction. The constituents of the KB medium may act to prevent these processes.