A reduction in L-type Ca(2+) current (I (Ca,L)) contributes to electrical remodeling in chronic atrial fibrillation (AF). Whether the decrease in I (Ca,L) is solely due to a reduction in channel proteins remains controversial. Protein tyrosine kinases (PTK) have been described as potent modulators of I (Ca,L) in cardiomyocytes. We studied alpha(1C) L-type Ca(2+) channel subunit expression and the regulation of I (Ca,L) by PTK in chronic AF using PTK inhibitors: genistein, a nonselective inhibitor of PTK, and 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo-3,4-d-pyrimidine (PP1), a selective inhibitor of src kinases. Furthermore, type-1 and type-2A protein phosphatase activity was measured with phosphorylase as substrate in whole-cell lysates derived from atrial tissue of AF patients. Right atrial appendages were obtained from patients undergoing open-heart surgery. Protein levels of alpha(1C) L-type Ca(2+) channel subunit were determined using Western blot analysis and normalized to the protein amounts of calsequestrin as internal control. The protein concentrations of alpha(1C) did not differ between AF and sinus rhythm (SR; alpha(1C)/calsequestrin: 1.0 +/- 0.1 and 1.2 +/- 0.2, respectively, n = 8 patients). In cardiomyocytes from patients in SR (n = 20 patients), genistein and PP1 both evoked similar increases in I (Ca,L) from 3.0 +/- 0.3 to 6.1 +/- 0.8 pA/pF and from 2.8 +/- 0.4 to 6.1 +/- 0.6 pA/pF, respectively. In cells from AF patients (n = 10 patients), basal I (Ca,L) was significantly lower. In this case, genistein lead to the same relative increase in I (Ca,L) as in SR cells (from 1.46 +/- 0.30 to 3.2 +/- 1.0 pA/pF), whereas no increase was elicited by PP1 suggesting impaired regulation of I (Ca,L) by src kinases in AF. Total and type 1 and type 2A-related phosphatase activities were higher in tissue from patients with chronic AF compared to SR (4.8 +/- 0.4, 2.1 +/- 0.2, and 2.7 +/- 0.4 nmol/mg/min and 3.6 +/- 0.4, 1.3 +/- 0.2, and 2.4 +/- 0.3 nmol/mg/min, respectively, n = 7 patients per group). Downregulation of I (Ca,L) in AF is not due to a reduction in L-type Ca(2+) channel protein expression. Indirect evidence for an impaired src kinase regulation of I (Ca,L) together with an increased phosphatase activity suggests that a complex alteration in the kinase/phosphatase balance leads to I (Ca,L) dysregulation in chronic AF.