Isolated tubule perfusion

Isolated tubule perfusion is an experimental technique in which a dissected renal tubule segment is supplied internally with a controlled solution while a separate bath contacts its outside. Fine micropipettes deliver and collect tubular fluid. Keeping luminal and external conditions separate lets investigators test transport across the epithelium of one identified segment.

For example, isolated rabbit thin descending limb segments were perfused in a 1970 study while bath osmolality was changed. Concentration of the collected fluid increased mainly through water leaving, rather than substantial salt entering. The apparatus converted a question about whole-kidney concentration into a direct test of one segment's membrane permeability.^[ref]

A simple mass balance estimates net water transport. If inflow and collected outflow are $Q_{\mathrm{in}}$ and $Q_{\mathrm{out}}$ for a segment of length $L$, then outward volume transport per unit length isassuming no leak or accumulation. Hypothetical flows of $20$ and $18nL/min$ through a $2\mathrm{mm}$ segment give $J_v=1\mathrm{nL}\mathrm{mi}{\mathrm{n}}^{-1}\mathrm{m}{\mathrm{m}}^{-1}$. A solute requires a separate balance involving concentration as well as flow.

Isolation provides experimental control while removing the segment from its normal blood supply, neighboring cells and systemic signals. It therefore complements renal micropuncture in an intact kidney: neither preparation alone supplies every part of the physiological explanation.