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. Action potential duration in LPS group was significantly shorter than control values (time to 50% repolarization: LPS, 169± 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 myoocytes from LPS group (-3.2±0.8 pA/pF), compared with respective values of control myocytes (-6.7±0.3 pA/pF, P<0.05). The ADM antagonist ADM-(22-52) reversed the shortened action potential duration 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 Gi protein inhibitor, pertussis toxin (PTX), the protein kinase A (PKA) inhibitor, KT-5720, and the specific 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 Gs-coupled to PTX- sensitive Gi-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.