An apparatus for varying the length of isolated heart muscle.

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Abstract
An apparatus utilizing the base of a microscope stand, a motor, and electrical circuitry was constructed to control and monitor length and force of isolated cardiac muscle. Myograms and muscle length changes were recorded simultaneously. Electrical stimulation during repetitive episodes of lengthening and shortening of cat papillary muscle and trabeculeae carnae led to immediate visualization of reproducible force-length curves recorded at low and high sensitivity. The effects of various inotropic interventions, including paired stimulation, were observable on segments of or on the entire dynamic force-length relationship. Similar studies were made on cyclically overstretched muscles where active contractile force development was on the descending limb of the Starling curve. The interaction of "heterometric" and "homeometric" auto-regulatory mechanisms was thus readily discernible in normal or "dilated" isolated cardiac muscle. Paired stimulation increased both contractility and diastolic compliance during periods of muscle lengthening and shortening. It is suggested that the apparatus can be useful in similar studies on force-length relationships for other types of muscle tissue.

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