Phosphatidic acid increases intracellular free ca2+ and cardiac contractile force. Xu, Yan-Jun, Vincenzo Panagia, Qiming Shao, Xi Wang, and Naranjan S. Dhalla. Division of Cardiovascular Sciences, St. Boniface General Hospital Research Centre, and Departments of Physiology and Anatomy, Faculty of Medicine, University of Manitoba, Winnipeg, Canada R2H 2A6
APStracts 3:0160H, 1996.
Phosphatidic acid increases intracellular free Ca2+ and cardiac contractile force. Am. J. Physiol. Although phosphatidic acid (PA) is mainly formed due to the hydrolysis of phosphatidylcholine by myocardial phospholipase D, its functional significance in the heart is not fully understood. The present study was designed to determine the effects of PA on intracellular free Ca2+ level ([Ca2+]i) in freshly isolated adult rat cardiomyocytes by using Fura-2/AM and free Fura-2 technique. Addition of PA at concentrations of 1 to 200 [mu]M produced a concentration-dependent increase in [Ca2+]i from the basal level of 117 +/- 8 nM; maximal increase in [Ca2+]i was 233 +/- 50 nM whereas EC50 for PA was 45 +/- 1.2 [mu]M. This increase in [Ca2+]i was abolished by the removal of extracellular Ca2+ with EGTA [ethylene glycol-bis ([beta]-aminoethyl ether) N,N,N',N'-tetraacetic acid] and was partially attenuated by Ca2+ channel blockerschannel blockers, verapamil or diltiazem. Preincubation of cardiomyocytes with cyclopiazonic acid and thapsigargin or with ryanodine (to deplete sarcoplasmic reticulum (SR) Ca2+) attenuated the PA-induced increase in [Ca2+]i by 66%, 37%, and 43%, respectively. Furthermore, the response of [Ca2+]i to PA was blunted by 2-nitro-4 carboxyphenylcarbonate, an inhibitor of phospholipase C, but was unaffected by staurosporine, a protein kinase C inhibitor. PA was also observed to induce Ca2+ efflux from the myocytes. In addition, an injection of PA (0.34 [mu]g/100 g body wt; iv) in rats produced a significant increase of the left ventricular developed pressure as well as the maximum rates of cardiac contraction and relaxation within 5 min. These data suggest that the PA-induced increase in [Ca2+]i in cardiomyocytes is a consequence of both Ca2+ influx from the extracellular source and Ca2+ release from the intracellular SR stores. Furthermore, these in vitro data suggest the possibility that PA may regulate [Ca2+]i and contractile parameters in the heart. 

Received 1 May 1995; accepted in final form 7 December 1995.
APS Manuscript Number H410-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 23 April 96