Characterization of the role of HCN channels in β3-adrenoceptor mediated rat bladder relaxation

OBJECTIVE The second messenger cAMP is involved in both β3 adrenoceptor (β3-AR) mediated detrusor relaxation and the kinetics of Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Here we characterized the effect HCN channel activation and possible interaction with β3-AR in bladder. MATERIALS AND METHODS Bladder tissues from Sprague-Dawley rats and Human organ donors were obtained for studying species-specific expression of HCN channels by real-time qPCR and Western Blot. Effect of β3-agonist on rat bladder strips (0.5 × 0.5 × 7 mm in size) was studied during activation and blockade of HCN channels by Lamotrigine and ZD7288, respectively. RESULTS Expression of all four genes encoding for HCN channels (HCN1-4) was detected separately in bladder mucosa and detrusor from human and rat bladders. Species based differences were evident from relatively higher expression of HCN4 isoform in human bladder and that of HCN1 in rat bladder. Western blot confirmed the findings at mRNA level. Cumulative application β3-AR agonist CL316,243 produced a concentration dependent decrease in resting tension of rat bladder strips expressed as integral of mechanical activity. Pre-incubation of HCN channel blocker ZD 7288 opposed the relaxant effect of CL316,243, whereas co-administration of lamotrigine with CL316,243 at equal molar concentrations caused an additive decrease in resting tension. Cumulative addition of ZD7288 and lamotrigine in absence of CL316,243 showed opposing effects on detrusor contractility. CONCLUSIONS Species-specific differences were noted in expression of HCN channels in bladder. Opposing effects ZD7288 and Lamotrigine in the action of β3-AR agonist demonstrate possible functional interaction of HCN channels and β3-AR in detrusor contractility.