Corticosteroids have been shown to increase the L-type Ca²⁺ current (I_CaL) in cardiomyocytes in vitro by activation of the mineralocorticoid receptor (MR). The signal transduction pathways involved, however, are incompletely understood. Here we demonstrate that the upregulation of I_CaL depends on the presence of insulin or IGF-1 and is sensitive to inhibition of the phosphatidylinositol 3-kinase (PI3-K) or the serum-and-glucocorticoid-induced proteinkinase (SGK-1). Left ventricular cardiomyocytes were isolated from female Wistar rats and investigated by the whole-cell patch-clamp technique after 24h incubation with corticosteroids alone, in combination with insulin or IGF-1 and/or in the presence of inhibitors of PI3-K or SGK-1. In the presence of 100nM insulin, dexamethasone (1mM) increased I_CaL (at 0mV) by 49% from -8.2±0.6 pApF^-1 (n=15) to -12.2±0.9 pApF^-1 (n=15, p<0.001), while dexamethasone alone had no effect (-8.2±0.7 pApF^-1, n=11). Co-incubation with 1mM dexamethasone and 10nM IGF-1 increased I_CaL by 42% (p<0.01). Insulin or IGF-1 alone did not affect I_CaL after 24h. Concentration-response relations revealed an EC₅₀ of 0.43nM and 4.7nM for IGF-1 and insulin, respectively. Similarly, incubation with aldosterone increased I_CaL in the presence of insulin but not in its absence. Coincubation with the PI3-K inhibitors PI-103 (0.5mM) or LY294002 (50mM) as well as coincubation with the SGK inhibitor GSK650394 (10mM) abrogated the dexamethasone/insulin-induced effect on I_CaL, while in the absence of dexamethasone/insulin, the inhibitors did not alter I_CaL. We conclude that corticosteroids and insulin/IGF-1 synergistically induce the increase in I_CaL and may constitute important cofactors in the regulation of cardiac function under physiological and pathophysiological conditions.