The effect of adrenomedullin on the L-type calcium current in myocytes from septic shock rats: signaling pathway

1 Department of Physiology and Institute of Cardiovascular Sciences and Medicine, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region of China, 2 Department of Pharmacy, Tongji Hospital of Tongji Medical College of Huazhong University of Science and Technology, Wuhan, China Submitted 13 March 2007 ; accepted in final form 27 August 2007 Adrenomedullin (ADM) is upregulated in cardiac tissue under various pathophysiological conditions, particularly in septic shock. The intracellular mechanisms involved in the effect of ADM on adult rat ventricular myocytes are still to be elucidated. Ventricular myocytes were isolated from adult rats 4 h after an intraperitoneal injection of lipopolysaccharide (LPS, 10 mg/kg). Membrane potential and L-type calcium current ( I Ca,L ) were determined using whole cell patch-clamp methods. APD in LPS group was significantly shorter than control values (time to 50% repolarization: LPS, 169 ± 2 ms; control, 257 ± 2 ms, P < 0.05; time to 90% repolarization: LPS, 220 ± 2 ms; control, 305 ± 2 ms, P < 0.05). I Ca,L density was significantly reduced in myocytes from the LPS group (–3.2 ± 0.8 pA/pF) compared with that of control myocytes (–6.7 ± 0.3 pA/pF, P < 0.05). The ADM antagonist ADM-(22-52) reversed the shortened APD and abolished the reduction of I Ca,L in shock myocytes. In myocytes from control rats, incubating with ADM for 1 h induced a marked decrease in peak I Ca,L density. This effect was reversed by ADM-(22-52). The G i protein inhibitor, pertussis toxin (PTX), the protein kinase A (PKA) inhibitor, KT-5720, and the specific cyclooxygenase 2 (COX-2) inhibitor, nimesulide, reversed the LPS-induced reduction in peak I Ca,L . The results suggest a COX-2-involved PKA-dependent switch from G s coupled to PTX-sensitive G i coupling by ADM in adult rat ventricular myocytes. The present study delineates the intracellular pathways involved in ADM-mediated effects on I Ca,L in adult rat ventricular myocytes and also suggests a role of ADM in sepsis. pertussis toxin; protein kinase A inhibitor Address for reprint requests and other correspondence: X.-H. Zhang, Dept. of Physiology, 4/F, Laboratory Block, Faculty of Medicine Bldg., The Univ. of Hong Kong, 21 Sassoon Rd., Hong Kong, S.A.R. China (e-mail: xh-zhang1030{at}hotmail.com )