The hyperpolarization-activated current If in ventricular myocytes of non-transgenic and β2-adrenoceptor overexpressing mice

In transgenic mice (TG4) overexpressing the human beta2-adrenoceptor (beta (2)-AR), unoccupied receptors are supposed to activate spontaneously the signalling cascade, leading to enhanced levels of cAMP. This second messenger shifts activation curves of the hyperpolarization-activated current I-f towards less negative potentials. Here, we characterize I-f of ventricular myocytes from non-transgenic littermate (LM) and TG4 mice and investigate whether I-f is modulated by spontaneous beta (2)-AR signalling. I-f was activated in whole-cell voltage-clamp experiments during test steps ranging from -65 mV to -135 mV (holding potential: -55 mV; 36 degreesC). In TG4 the maximum amplitude was fivefold larger than in LM myocytes (-1.10 +/-0.11 pA/pF vs. -0.22 +/-0.04 pA/pF at -135 mV), and the potential for half-maximum I-f current V-10.5) was less negative (-100.5 +/-1.0 mV in TG4 vs. -108.4 +/-2.6 mV in LM). ((-)-Isoproterenol (1 muM) shifted V10.5 of LM myocytes by 10.4 mV towards less negative potentials but had no significant effect in TG4. However, the inverse beta2-AR agonist ICI 118,551 (300 nM) shifted V-10.5 of TG4 myocytes to values observed in LM under control conditions, suggesting a relation to spontaneously active beta (2)-ARs. Enhanced expression of hyperpolarization-activated and cyclic nucleotide gated channels (HCN) could contribute to increased maximum I-f amplitude in TG4 myocytes. Semi-quantitative RT-PCR analysis demonstrated a 1.8-fold elevation of HCN4 mRNA and no significant change for HCN2 mRNA in TG4 ventricle. Cardiac hypertrophy was not detected in TG4 mice investigated here. We conclude that spontaneous beta (2)-AR signalling in hearts of TG4 mice shifts If current-voltage relation towards less negative potentials. Increased maximum If amplitude in TG4 myocytes is in line with enhanced expression of HCN channels. Both mechanisms could contribute to larger inward current at physiological diastolic potentials.