A comparison of contractile function between papillary muscles and isolated myocytes from the same human hearts

The contractile function in response to increasing extracellular calcium and isoprenaline has been investigated using papillary muscles from seven human hearts (failing and non-failing) obtained at the time of transplant. Myocytes have been enzymatically isolated from adjacent myocardium, and their contractile characteristics measured in parallel to those of the intact papillary muscle. Similar experiments were performed on rabbit hearts, but in this case separate animals were used for the papillary muscles and cells. For human myocardium, the EC50 (concentration for half-maximal effect) for calcium, and the percentage increase in amplitude between basal (1.3mM) and maximally activating calcium, was similar between papillary muscle and cells. Variability of absolute responses was 4-5 times greater for papillary muscle than for myocytes. Isoprenaline accelerated the time to peak tension, time to 50% relaxation and time to 90% relaxation in papillary muscle; a similar effect on shortening was seen for time to peak tension and time to 50% relaxation in cells. Time to peak tension and time to 50% relaxation were not significantly different between the human preparations with isoprenaline, but the time to 50% relaxation with high calcium was faster for myocytes than for papillary muscle (p less than 0.05). Isoprenaline induced after-contractions in some cells, prolonging the time to 90% relaxation. This was not observed in papillary muscle preparations. Papillary muscles were less sensitive to the stimulatory effects of isoprenaline than cells taken from the same hearts. EC50 values were higher (p less than 0.05) and increases in contraction were lower (p less than 0.05) in whole tissue. This difference between cells and papillary muscle was not seen in rabbit hearts.(ABSTRACT TRUNCATED AT 250 WORDS)