Abstract
Length-tension relationships observed in the rat ventricle strip can be expressed by Starling's Law of the Heart. Similarities and differences in these relationships between cardiac and skeletal muscle have been observed. During isometric contraction the total tension exerted by such strips is the sum of resting tension and of systolic tension. With some drugs which affected resting tension, these latter two parameters were found to be reciprocally related. Alternation of the heat produced by various drugs was observed to be frequency-dependent. At high frequencies the onset of this effect was almost immediate while longer times were required at lower frequencies. The available evidence suggests that this effect is mediated by the action of drugs on excitability and refractory period of heart muscle. The inverse relationship between heart rate and refractory period may account for the alternation's frequency-dependent characteristics. The time of onset of the alternation was found to depend also on drug concentration and on the ionic composition of the medium. In addition, the character of the alternation was influenced by various inotropic drugs, by frequency, by stimulus strength and by periods of rest. In other experiments frequency-dependent augmentation of cardiac contractility associated with periods of rest and with periods of rapid stimulation and rest was observed. In addition, augmentation of contractility with extrasystoles was noted. These have been previously reported in other heart muscle preparations. However, positive treppe, also previously reported, was not seen in the rat ventricle strip except at very low frequencies. The effects of various drugs on these phenomena were studied.
Footnotes
- Received June 24, 1957.
JPET articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|