ELECTRICAL STIMULATION OF ISOLATED HEART PREPARATIONS FROM PERIPLANETA AMERICANA' By J. FRANKLIN YEAGER Senior entomologist. Division of Control Investigations, Bureau of Entomology and Plant Quarantine, United States Department of Agriculture INTRODUCTION

In order to study adequately the effects of insecticides upon the various tissues and functional processes of insects, particularly insects which it is desired to control, it is necessary to have more knowledge of the manner in which the tissues and organs of insects function under normal and abnormal conditions. This paper is a report of experiments which yielded further information regarding the functional processes of the insect-cardiac mechanism. In utilizing the mechanocardiographic method of studying the effects of nicotine and other solutions upon a perfused, isolated heart preparation from the American cockroach (Periplaneta americana (L.)) it is necessary to consider the effects of possible contractions by other than cardiac-muscle fibers. In addition to the cardiac-muscle fibers, the cockroach isolated heart preparation previously used UJ 6y 6j 7y possessed alary muscles (of the heart), dorsal body muscles, and, in the thoracic region, especially of heart preparations from the adult insect, portions of the muscles involved in the somatic wing mechanisms. It has been shown previously (4) that the systolic rise of curve m the mechanocardiogram taken from the whole isolated heart preparation of Periplaneta americana is often immediately preceded by a sudden depression, the presystolic notch. It has been suggested (^), on the basis of certain unpublished evidence, that the presystoUc notch might result from an increased intracardiac pressure (hydrostatic) produced by a propagated wave of heartbeat in a cardiac region other than that from which the mechanocardiographic record is being taken. Although under certain conditions the presystolic notch may originate in this manner, the possibihty still remains that under other conditions it might originate from a rhythmic presystoHc contraction of the alary-muscle fibers of the heart, especially in the segment from which the record is being made. If the presystohc notch originates m accordance with this hypothesis, it should be possible to obtain supportive evidence by artificially stimulating the isolated heart preparation, especially the alary-muscle fibers in the segment in which the heart lever is attached. . . The experiments herein reported were performed with this m mmd, but also with the general objective of determining how the isolated heart preparation of Periplaneta americana responds to single and repeated faradic stimuh and how these responses compare with the better known results of artificial stimulation of the vertebrate heart.