This study aimed at characterizing expression and the functional role of the Gjb6 gene, encoding for connexin 30 (Cx30) protein, in the adult mouse heart.The expression of the Gjb6 gene in the mouse heart was investigated by RT-PCR and sequencing of amplified cDNA fragments. The sites of Gjb6 expression were identified in the adult heart using transgenic mice with reporter genes (Cx30(LacZ/LacZ) and Cx30(LacZ/LacZ)/Cx40(EGFP/EGFP) mice), as well as anti-HCN4 (hyperpolarization activated cyclic nucleotide-gated potassium channel 4) or anti-connexin antibodies. Cine-magnetic resonance imaging and telemetric ECG recordings were used to evaluate the impact of Cx30 deficiency on cardiac physiology. Gjb6 was shown to be expressed in the sinoatrial (SA) node of the adult mouse heart. Eighty from 100 nuclei on average were LacZ-positive in the SA node of Cx30(LacZ/LacZ) mice. No significant LacZ expression was seen in other cardiac tissues. Cx30 protein was identified in low abundance in the SA node of wild-type mice, as indicated by immunofluorescence experiments. Telemetric ECG recordings indicated that Cx30-deficient mice displayed a mean daily heart rate (HR) that was 9% faster than that measured in control mice (572 +/- 38 b.p.m. vs. 524 +/- 23, P < 0.05). This moderate tachycardia was still observed after inhibition of the autonomic nervous system, demonstrating that Cx30 deficiency resulted in changes in the intrinsic electrical properties of the SA node. Consistent with this hypothesis, Cx30(LacZ/LacZ) displayed a significant reduction of SDNN (standard deviation of the interbeat interval) compared with control mice. Increase of both the cardiac index (20%) and the end-diastolic volume to body weight ratio (16%) with no deficiency in ejection fraction or stroke volume were observed in mutant mice. An increase in cardiac index was interpreted as being a direct consequence of high HR, whereas large end-diastolic volume may be an indirect consequence of prolonged high HR.Cx30 is functionally expressed, in low abundance, in the SA node of the adult mouse heart where it participates in HR regulation.