Effects of trypsin on large-conductance Ca²⁺-activated K⁺ channels of guinea-pig outer hair cells

High-conductance Ca²⁺-activated K⁺ (BK_Ca) channels from isolated adult guinea-pig outer hair cells were studied in inside-out membrane patches. They had a 300 pS unitary conductance and were inhibited by tetraethyl ammonium (1 mM), iberiotoxin (33 nM) and charybdotoxin (50 nM). In symmetrical 144 mM KCl their K⁺ permeability (P_K) was 5.4×10⁻¹³ cm³/s; this was reduced to around 4.5×10⁻¹³ cm³/s with 160 mM Na⁺ in place of K⁺ on either internal or external membrane surface. BK_Ca channels from trypsin-isolated hair cells had a high open probability, that depended on both membrane voltage (16 mV/e-fold change) and the concentration of calcium ions at their intracellular surface ([Ca²⁺]_i). The Hill coefficient was 3–4. About 50% of BK_Ca channels from mechanically isolated outer hair cells had similar characteristics; the remainder had the same high conductance but a low open probability. Trypsin (<0.5 mg/ml) applied to the intracellular face of these ‘inactive’ channels markedly increased their open probability. It is possible that exposure to trypsin during cell isolation removes an inactivating β subunit. This would account for the absence of ‘inactive’ BK_Ca channels in trypsin-isolated cells.