Comparison of the effects of inotropic interventions on isometric tension and shortening in isolated ferret ventricular muscle

In this study, we have compared the effects of a range of positive and negative inotropic interventions on isometric tension and shortening at low load in isolated ferret papillary muscles. It was found that the majority of interventions produced broadly similar effects on tension and shortening. Positive inotropic interventions which behaved in this way included increasing extracellular calcium and increasing frequency at moderate stimulation rates. Acidosis, the addition of cyanide, and metabolic blockade produced by the addition of iodoacetic acid and cyanide, all produced negative inotropic effects which showed a similar pattern. For all these interventions, it was noted that the fractional effect on tension was 1.5-2 times larger than the fractional effect on shortening. This was attributed to the shape of the tension-length relation for cardiac muscle in different inotropic states. When frequency was increased to a high rate, tension rose to an initial peak, but then showed a marked decline, while shortening rose to a new level and was then well maintained. When protocols consisting of varying amounts of isometric and isotonic contraction were applied, the amount of the secondary decline for both tension and shortening was related to the period spent doing isotonic contractions, and hence to the energy consumption of the muscle. Thus the difference in the behaviour of tension and shortening under these conditions can be accounted for by the lower energy requirements of shortening, rather than by other factors. Over the range of inotropic interventions studied, peak shortening velocity was a more sensitive index of contractility than shortening, being roughly comparable to isometric tension.(ABSTRACT TRUNCATED AT 250 WORDS)