Sympathetic innervation alters activation of pacemaker current (I-f) in rat ventricle

1. Pacemaker current (I-f) exists in both neonatal and adult ventricles, bu t activates at more negative voltages in the adult. This study uses whole-c ell patch clamp to investigate the factors that may contribute to the matur ational shift of I-f, comparing neonatal rat ventricular myocytes that were cultured fur 4-6 days either alone, in co-culture with sympathetic nerves, or with neurotransmitters chronically present in culture. 2. I-f recorded from nerve-muscle co-cultures had a significantly more nega tive and shallower activation-voltage relation than that from control muscl e cultures, which was reflected in the midpoint potential (V-50) and slope factor (K) of activation. This effect of innervation was prevented by the s ustained presence in the culture of the alpha(1)-adrenergic antagonist praz osin (P-z) at 10(-7) hi. 3. In parallel experiments, myocytes treated with noradrenaline (NA) at 10( -7) M or neuropeptide Y (NPY) at 10(-7) M during culture had the same I-f a ctivation as control cells, but cells treated with NA and NTP together had a significantly more negative and shallower activation curve. Maximum condu ctance and reversal potential were unchanged. 4. The effect of chronic exposure to NA + NPY was pre-vented by the sustain ed presence of either Pt or the NPY Y-2 selective antagonist T-4-[NPY(33-36 )](4) (3.5 x 10(-7) M) in the culture, indicating a requirement for both al pha(1)-adrenergic and NPP Y-2 activation. 5. Substituting NA with the alpha(1A)-adrenergic selective agonist A61603 ( 5-10 x 10(-9) M), in the presence of NPY, did nut alter alpha(1B)- suggesti ng the involvement of alpha(1B)- rather than alpha(1A)-adrenoceptors. Furth er, sequential exposure to NPT followed by NA was effective in reproducing the action of chronic simultaneous exposure to these agonists, but sequenti al exposure to NA followed LSI NPY was ineffective. 6. The results are consistent with past studies indicating that NPY affects the functional expression of the alpha(1B)-adrenergic cascade and suggest that sympathetic innervation induces a negative shift of I-f in ventricle v ia a combined action at alpha(1B)-adrenergic and NPY Y-2 receptors. This ef fect of innervation probably contributes to the developmental maturation of I-f activation.