Figure 14.

Mixing volumes determined by nystatin-mediated currents are predictably changed by osmolarity gradients. (A) Representative record of access resistance and capacitance when extracellular osmolarity is doubled for 80 s by 300 mM extracellular polyethylene glycol (PEG; 500 MW). Otherwise, solution compositions were standard, with no Na. Access resistance, which includes longitudinal myocyte resistance, increases as expected for a reduction of the myocyte diameter. Note that resistance does not return to baseline after removing PEG. (B) Membrane current transients in the presence of nystatin when 120 mM NMDG is replaced with 120 mM Na. Osmolarity of the pipette solution was increased to 400 mosM/liter with 100 mM PEG. After recording the initial responses to Na changes, the extracellular solution was switched to a hyperosmotic solution containing 220 mM PEG. Responses to cation substitution were recorded again and compared with the initial currents. (C) With 100 mM hyperosmotic pipette solution, the τ of current decay is 25 ± 2 s (n = 5) versus 12 ± 2 s (n = 5) in the 220 mM hyperosmotic solution. Mean τ values with isosmotic (290 mosM/liter) solutions on both sides amounted to 20 ± 3 s initially and did not change significantly after 800 s of recording time (not depicted, n = 4). Error bars indicate SEM.