Simple Passive Device to Ensure Isotonic Contraction of Intact Tubular or Spherical Organs

Abstract

Although both isometric (constant length) and isotonic (constant force) experiments are required to completely characterize muscle function, isotonic experiments are currently impossible to perform on intact, cyclically contracting structures such as cardiac ventricles and lymphatic vessels without complex, expensive equipment. We therefore designed a novel, passive device for isotonic experiments which, unlike traditional wire myographs, can be used to characterize the mechanics of intact spherical or tubular organs without distorting their natural geometries. An isolated lymphatic vessel or cardiac ventricle is cannulated and submerged in APSS. The outlet is connected to a vertical tube whose bottom is level with the tissue. Transmural pressure (P) is determined by the height of the solution column in the tube. Careful selection of the tube shape so that the top is narrower than the bottom ensures that decreases in radius (r) are balanced by increases in P, keeping wall tension (T) constant. For example, the equation describing the shape of the tube for lymphatic vessel experiments can be derived by assuming conservation of mass and Laplace's Law (T = P x r). In this case, the width if the tube w(h) is inversely proportional to the cube of its height h. Unlike computer-based, piston-driven systems, this passive device is simple, inexpensive, easy to construct, and self-calibrating. w(h): width of the tube as a function of solution column's height l: length of the vessel T: pre-set wall tension t: thickness of the tube h: height of the solution column