Calcium uptake by mechanically disaggregated heart muscle

Effects of acetylcholine, isoprenaline, phenylephrine and dibutyryl-cAMP on the contractility of isolated myocytes of the rat heart ventricles

Single ventricular myocytes were isolated by enzymatic dissociation from the rat heart. The myocytes showed two types of spontaneous contractions: action potential dependent 'electric' contractions which occurred as short bursts, and slowly propagating contraction waves, i.e. phasic contractions. Electric contractions were normally found only in some of the myocytes, but in about 85% of the cells these could be induced by isoprenaline (ISO), phenylephrine (PHE), dibutyryl-cAMP (DB-cAMP) or acetylcholine (ACh). The inducing potency of the drugs was ACh greater than PHE greater than ISO greater than DB-cAMP. When electric contractions were blocked with verapamil, the frequency of phasic contractions was increased by ISO and DB-cAMP, whereas PHE and ACh had not this effect. In 45 min, ISO increased the frequency tenfold. High concentrations (10(-4) and 10(-5) M) of ISO lead to steady contracture of the myocytes in about 40% of the cells. The effect of DB-cAMP was weaker being only about 20% of that of ISO and it never caused steady contractures. The results suggest that alpha receptor activation by PHE does not affect the intracellular calcium movements but that ISO through beta receptor activation and DB-cAMP directly may increase the rate of calcium accumulation by sarcoplasmic reticulum (SR).

Characterization of contractions in mechanically and enzymatically isolated myocardial cells from the rat heart ventricles

Single spontaneously beating myocardial cells were prepared by mechanical or enzymatical isolation. Using a TV camera and videotape recorder, beating parameters were characterized in different buffer solutions in regard to temperature and integrity of sarcolemma, which was tested by ability of cells to exclude a vital stain, Evans blue. The mechanically disaggregated cells were totally permeable to this stain, but about 60% of the enzymatically isolated myocytes were able to exclude the stain. The myocytes prepared in the presence of calcium were most tolerant to physiological concentration of calcium. Contractions were mainly of phasic type where the velocity of contraction wave was very slow (50-330 microns/s). The velocity was not correlated to the beat rate. Temperature dependence of the velocity was positively correlated with the integrity of sarcolemma. Beat rate was inversely related to the integrity of sarcolemma being slowest in calcium tolerant enzymatically isolated cells. Beat rate of calcium sensitive myocytes was higher but unstable and the temperature dependence of the rate was steeply reduced and even reversed during the incubation. Some calcium tolerant myocytes generated occasionally an electric type of contraction which came in bursts and was characterized by synchronous sarcomere shortening without any contraction wave.

Characterization of contractions in mechanically disaggregated myocardial cells from the rat

Myocardial cell groups, mechanically disaggregated from the rat heart, beat spontaneously in a medium which resembles intracellular fluid. The cells, initially functioning in an uncoordinated way, i.e. each cell in its own rhythm, gradually become coordinated as a function of time. After 90 min of incubation at 15 degrees C all the cell groups were functioning in a coordinated way. The initiation of coordination was temperature-dependent. Lowering the temperature from 15 degrees C to 10 degrees C significantly delays the coordination, only 75% of the cell groups being coordinated after 90 min. The contraction wave in coordinated cell groups was very slow; 90 +/- 7 micrometers/s at 20 degrees C, 100 +/- 6 micrometers/s at 30 degrees C and 100 +/- 7 micrometers/s at 35 degrees C. The velocity of contraction waves was not significantly temperature-dependent, suggesting a neonenzymatical type of propagation. It also seems too slow to be an electrical event. Apparently the contraction waves are propagated by diffusion of Ca2+. The addition of caffeine to the medium hastened the coordination of contractions, but Na-azide strongly inhibited it. The results indicate that the sarcoplasmic reticulum functions in the coordination of contractions, probably by releasing Ca2+ for the activation of the myofilaments.

Calcium transport and contractile activity in dissociated mammalian heart cells

A homogeneous population of dissociated myocytes can be obtained by enzymic perfusion of adult rabbit hearts. The external membrane of the dissociated cells is freely permeable to electrolytes and is electrochemically shunted. Nevertheless, in the presence of 0.2--0.6 muM Ca2+, the myocytes undergo phasic contractions. Within the range of Ca2+ concentrations eliciting phasic contractions, the dissociated myocytes exhibit Ca2+ uptake activity that is sustained by endogenous ATP in the presence of oxygen and metabolites, or by hexogenous ATP in the presence of oligomycin and antimycin. The uptake is greatly enhanced by oxalate, an effect that is attributed to the activity of sarcoplasmic reticulum. Addition of the calcium ionophore A23187 prevents the occurrence of both Ca2+ accumulation and phasic contractions. In the presence of the ionophore the myocytes remain relaxed at Ca2+ concentrations (less than 1.0 mM) normally sustaining phasic contractile activity, and undergo irreversible contracture at Ca2+ concentrations (greater than 1.0 muM) sufficient to produce direct activation of the myofibrils. It is concluded that Ca2+ transport by sarcoplasmic reticulum is directly related to the occurrence of phasic contractions in dissociated myocytes prepared from rabbit heart. On the other hand, metabolic activity is required for ATP regeneration.

The relationship between calcium flux and beating in hyperpermeable heart muscle cells

Beating hyperpermeable myocardial fragments, suspended in fresh medium, show a Ca uptake rate which greatly exceeds the release rate. Under these conditions, Ca influx and contraction frequency co-vary with temperature. Ca efflux, however, is not temperature responsive. In addition, influx, but not efflux, is ATP-dependent. This implies that influx, but not efflux, is a controlled process, the activity of which has the same determinants as does contraction. However, under conditions approximating the steady state, the Ca influx rate was found to decrease, while the efflux rate increased. This suggests the possibility, at least, that the source of contraction-activating Ca can vary with the prevailing conditions.