AJP - Regulatory, Integrative and Comparative Physiology, Vol 258, Issue 1 112-R119, Copyright © 1990 by American Physiological Society
Basal membrane uptake in potassium-secreting cells of midgut of tobacco hornworm (Manduca sexta)
A. C. Chao, A. R. Koch and D. F. Moffett
Department of Zoology, Washington State University, Pullman 99164.
Basal membrane voltage (Vb), intracellular K+ activity [(K+)i], and
short-circuit current (Isc) were measured in isolated posterior midguts of
Manduca sexta wherein Isc is a measured of active secretion of K+ from
blood into lumen. When bathed in 32 mM K+ and exposed to 100% O2, average
values were Isc = 244 microAmp/cm2, Vb = -33.1 mV, and (K+)i = 88.6 mM. The
electrochemical gradient across the basal membrane (d mu) averaged +5.8 mV
(a gradient favorable for K+ entry). Exposure to 5% O2 led to a new steady
state in which Isc = 71 microAmp/cm2, Vb = -18.7 mV, and (K+)i = 99.4 mM.
During hypoxia, d mu averaged -9.9 mV (a gradient unfavorable for K+
entry). When the external bathing solution was 10 mM K+, comparable values
were, for 100% O2, Isc = 139 microAmp/cm2, Vb = -56.1 mV, (K+)i = 72.2 mM,
and d mu = +3.6 mV, and in 5% O2 the values were Isc = 28.3 microAmp/cm2,
Vb = -43.7 mV, (K+)i = 76.1 mM, and d mu = -10.2 mV. The failure of
cellular K+ to fall during prolonged hypoxia is evidence for a
thermodynamically active basal K+ uptake process.
