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

Biophysical properties of A-type K(+) currents (I(A)) in myenteric neurons from guinea-pig small intestine were studied. I(A) was present in both AH- and S-type myenteric neurons. Reduction of external Ca(2+) did not affect the current. 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 homogeneous group based on density of I(A). AH-type neurons were subdivided into two groups with current densities of 9.4+/-4.3 and 25.4+/-4.3 pA/pF. All other biophysical properties of the current were not statistically different for AH- and S-type neurons. Steady-state activation and inactivation curves 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 approximately -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 fell between 41.5 and 11 ms at test potentials between -20 and 50 mV. Time constants 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. Addition of Zn(2+) in the external solution resulted in a concentration-dependent shift of the activation and inactivation curves in the depolarized direction. Zn(2+) 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 Zn(2+) with a pK of 7.3-7.4. The effects of Zn(2+) were interpreted as not being due to surface charge screening, because the affinity of Zn(2+) for its binding site on the A-channel was estimated to be between 170 and 312 microM, while the background concentration of Mg(2+) was 10 mM. The enteric nervous system is perceived as an independent integrative nervous system (brain-in-the-gut) that is responsible for local organizational control of motility and secretory patterns of gut behavior. AH- and S-type neurons are synaptically interconnected to form the microcircuits of the enteric nervous system. The results suggest that I(A) is a significant determinant of neuronal excitability for both the firing of nerve impulses and the various synaptic events in the two types of neurons.