The purpose of this grant application is to study mammalian heart cell function in vitro employing as the major tool of investigation a laser microbeam in combintion with various additional methodologies of tissue culture, electron microscopy, electrophysiology, and pharmacology. The specific objectives of the application are (1) to perform a detailed ultrastructural analysis on irradiated mitochondria and cells in various states of laser induced contractility; (2) to investgate the mechanism of laser radiation interaction with the heart cells by performing action spectrum analyses, ultrastructural studies, frequency shifting by second harmonic generation, and therma analysis; (3) to inquire into the nature of the induced contractility states by employing electrophysiological monitoring of membranes, pharmacological modification of cellular activity, determination of Ca ions changes, and an inquiry into the pacemaker and non-pacemaker activity of cells.