Amphiuma

Abstract: Experiments were performed in the distal tubule of the doubly-perfused kidney of Amphiuma to determine active and passive forces, involved in the transport processes of potassium, sodium and chloride. Ion-sensitive microelectrodes and conventional microelectrodes were applied to estimate intracellular ion activities, cell membrane potentials and net flux of potassium and chloride under control conditions and during inhibition of active transport. Sodium chloride cotransport, located in the luminal cell membrane is postulated, based on the following observations:

Abstract: Intracellular potassium activity (aKi) has been determined in absorptive cells lining the villi of isolated, stripped proximal segments of Amphiuma small intestine. With single-barreled liquid ion-...

Abstract: An increase in extracellular Ca concentration causes the membrane of giant red cells of the salamander,Amphiuma means, to undergo a marked, transient hyperpolarization. This hyperpolarization is caused by an increase in K permeability of the membrane as judged from the K sensitivity of the membrane potential and from the rate of K loss under influence of raised extracellular Ca concentration. At constant external pH, the induction of hyperpolarization by increased extracellular Ca has a relatively well-defined threshold concentration. Furthermore the phenomenon is of an “all or none” type with most of the cells having membrane potential values either in the normal range (about −15 mV) or in the range −40 to −70 mV. Shortly after suspension in Ringer's with 15mM Ca, most if not all of the individual cells are hyperpolarized. Upon continued exposure (5–20 min) to the higher Ca concentration the membrane potential returns to the normal value in a fashion compatible with an “all or none” response. The observed Ca effect is sensitive to the pH of the suspending medium. At pH 6.2 the response is absent whereas the hyperpolarization is markedly stronger at pH 8.2 than at pH 7.2. It is argued that a reliable transport number for K under influence of Ca cannot be estimated from the slope of membrane potentialvs. log (extracellular K concentration). This is probably related to the fact that the membrane potentials of the cells in the population do not stay constant in time. The above phenomenon is compared with the Ca-induced K permeability in poisoned human red cells or red cell ghosts. It is important to note that the cells employed in the present study are neither poisoned nor mechanically disrupted. This study emphasizes that the role of Ca in regulating cell membrane permeability to K seems to be a general feature.

Abstract: The erythrocyte ofAmphiuma means was chosen as a model for elucidation of membrane properties of red cells because the large size of this cell permitted direct measurements of plasma membrane potential. In the 30-sec period following micropuncture and withdrawal of the electrode the plasma membrane reseals and hyperpolarizes to a value of about −50 mV. The hyperpolarization is followed by a gradual return to the unperturbed potential of −15 mV. The magnitude of the hyperpolarization is strongly reduced by an increase in extracellular K concentration and is therefore related to an increase in relative K permeability. The transference number for K is calculated to have a maximal value of about 0.6. However, it is not yet clear whether the hyperpolarization can be solely attributed to a rise in K permeability, or whether there is a concomitant decline in Cl permeability as well. The magnitude of the hyperpolarization is unaffected by the presence of either ouabain or oligomycin.