A-type potassium current in myenteric neurons from guinea-pig small intestine

Biophysical properties of A-type K+ currents (I-A) in myenteric neurons fro m guinea-pig small intestine were studied I-A was present in both AH- and S -type myenteric neurons. Reduction of external Ca2+ did not affect the curr ent. Current density was 13.5 +/- 10.2 pA/pF in 68 AH-type neurons and 23.4 +/- 8.2 pA/pF in 31 S-type neurons. S-type neurons appeared to be a homoge neous group based on density of IA AH-type neurons were subdivided into two groups with current densities of 9.4 +/- 4.3 and 25.4 +/- 4.3 pA/pF. All o ther biophysical properties of the current were not statistically different for AH- and S-type neurons. Steady-state activation and inactivation curve s showed half-activation potentials at -7 mV (k= 15.0 mV) and -86 mV (k= 11 .5 mV). The curves overlapped at potentials near the resting potential of a pproximately -55 mV. Time constants for activation ranged from 3.6 to 0.52 ms at test potentials between -20 and 50 mV. Inactivation time constants fe ll between 41.5 and 11 ms at test potentials between -20 and 50 mV. Time co nstants for recovery from inactivation fit a double-exponential curve with fast and slow recovery times of 11 and 550 ms. 4-Aminopyridine suppressed I -A when it was activated at -20 mV following a pre-pulse to -110 mV. Additi on of Zn2+ in the external solution resulted in a concentration-dependent s hift of the activation and inactivation curves in the depolarized direction . Zn2+ slowed the activation and inactivation kinetics of I-A by factors of 3.3- and 1.2-fold over a wide range of potentials. Elevation of external H + suppressed the effect of Zn2+ with a pK of 7.3-7.4. The effects of Zn2+ w ere interpreted as not being due to surface charge screening, because the a ffinity of Zn2+ for its binding site on the A-channel was estimated to be b etween 170 and 312 mu M, while the background concentration of Mg2+ was 10 mM. The enteric nervous system is perceived as an independent integrative nervo us system (brain-in-the-gut) that is responsible for local organizational c ontrol of motility and secretory patterns of gut behavior. AH- and S-type n eurons are synaptically interconnected to form the microcircuits of the ent eric nervous system. The results suggest that I-A is a significant determin ant of neuronal excitability for both the firing of nerve impulses and the various synaptic events in the two types of neurons. (C) 2000 IBRO. Publish ed by Elsevier Science Ltd.