A new method for exchange of perfusion fluid during in vivo micropuncture

The first step in the tubuloglomerular feedback (TGF) mechanism is postulated to be the sensing of changes in tubular NaCl concentration by the macula densa (MD) cells. Despite this, few in vivo studies using different tubular NaCl concentrations administered to the MD site have been completed. Methodological problems associated with retrograde perfusion might possibly explain this. In the present study we present a modification of a method used in in vitro tubular perfusion experiments which makes it possible to determine the TGF response using retrograde perfusion with a single perfusion pipette. The system is based on the use of a fluid exchange pipette introduced through a chamber in the holder of the perfusion pipette all the way to the tip of this pipette. The perfusion flow was regulated by a pressure head, and by regulating the outflow from the chamber. The flow was determined for different tip diameters and for different perfusion pressures. Using a tip diameter of 4 microns, the flow through the tip was in the range 18-25 nL min-1 at a pressure of 60-70 mmHg. The fluid-exchange pipette was tested in vivo by measuring proximal tubular stop-flow pressure while perfusing the macula densa region with different NaCl concentration.