Since integrins are thought to function as mechanoreceptors we studied whether they could mediate mechanical modulation of L-type Ca²⁺ channel currents (I_Ca) in guinea-pig cardiac ventricular myocytes (cvm). Cvm were voltage clamped with 280 ms pulses from -45 to 0 mV at 0.5 Hz (1.8 mM [Ca²⁺]_o, 22 °C). Five minutes after whole-cell access (designated as 0 min) peak I_Ca was determined from an I-V curve. Additional recordings were made after 5, 10 and 15 min. At control, I_Ca was not stable, but ran down during these periods. This run-down of I_Ca was attenuated by soluble fibronectin (FN) and was changed to an enhancement of I_Ca when cvm were attached to FN-coated coverslips. Soluble peptide containing FN's integrin binding sequence Arg-Gly-Asp (RGD motif) did not modulate I_Ca, however, I_Ca increased in stimulated cmv attached to RGD-peptide-coated coverslips. The effect was not specific to integrins, because attachment to poly-D-lysine-coated coverslips also augmented I_Ca in stimulated cvm. Augmentation of I_Ca by immobilized FN required rhythmical contraction of attached cvm, because it was attenuated without electrical stimulation and after cell dialysis with the calcium chelator BAPTA. Furthermore, contraction-induced augmentation of I_Ca in FN-attached cvm was sensitive to inhibition of PKC (Ro-31-8220), inhibition of tyrosine kinase activity (herbimycin A) and cytoskeletal depolymerization (cytochalasin D, colchicine). We attribute augmentation of I_Ca to the activation of signaling cascades by shear forces that are generated when cvm contract against attachment; in vivo similar signals may occur when cvm contract against attachment of integrins to the extracellular matrix.