Dale J. Benos

Streaming potentials across cloned epithelial Na+ channels (ENaC) incorporated into planar lipid bilayers were measured. We found that the establishment of an osmotic pressure gradient (Deltapi) across a channel-containing membrane mimicked the activation effects of a hydrostatic pressure differential (DeltaP) on alphabetagamma-rENaC, although with a quantitative difference in the magnitude of the driving forces. Moreover, the imposition of a Deltapi negates channel activation by DeltaP when the Deltapi was directed against DeltaP. A streaming potential of 2.0 +/- 0.7 mV was measured across alphabetagamma-rat ENaC (rENaC)-containing bilayers at 100 mM symmetrical [Na+] in the presence of a 2 Osmol/kg sucrose gradient. Assuming single file movement of ions and water within the conduction pathway, we conclude that between two and three water molecules are translocated together with a single Na+ ion. A minimal effective pore diameter of 3 A that could accommodate two water molecules even in single file is in contrast with the 2-A diameter predicted from the selectivity properties of alphabetagamma-rENaC. The fact that activation of alphabetagamma-rENaC by DeltaP can be reproduced by the imposition of Deltapi suggests that water movement through the channel is also an important determinant of channel activity.

This article summarizes the comments made in the introductory session of the Medical Physiology course taught at the University of Alabama School of Medicine. The main thesis is that learning physiology is easier when fundamental principles are first delineated. Four general principles of physiology (mass balance, force-flow, capacitance, and equilibrium) are discussed. Clinical medicine becomes more comprehensible when these basic principles are understood. Cellular Physiology is taught first because it forms the conceptual basis for what follows in the course. The idea that the root of clinical medicine lies in the basic sciences is emphasized.