Fig. 1. Schematic representation of the device utilized for the simultaneous measurement of twitch contractile force and 3[H]- norepinephrine release. Device components are identified by numerals: 1 , water-jacketed chamber (10 ml capacity); 2 , water jacket inlet and outlet; 3 , oxygenation port (through sintered glass); 4 , chamber drain port; 5 , left atrial preparation; 6 , tissue support rod; 7 , platinum stimulating electrodes; 8 , stimulator; 9 , stainless steel attachment between tissue and force-displacement transducer; 10 , force-displacement transducer; 11 , strain gauge amplifier; 12 , computer; 13 , penwriter. Heated fluid is pumped into the water jacket inlet and exits to provide a stable bath temperature. Oxygen is delivered at constant flow through a separate oxygenation port containing a sintered glass fixture to generate small oxygen bubbles surrounding the tissue. The drain port allows fluid to be removed from the chamber as needed. The tissue is mounted between a fixed point on the tissue support rod and a stainless steel wire attached to the force-displacement transducer. The atrial preparation is located between two parallel stimulating electrodes connected to a stimulator. Force of contraction, after being detected by the force transducer, is amplified and displayed on the video monitor of a computer and a penwriter. 3[H]-norepinephrine tracer is added to the chamber liquid to label sympathetic nerve endings; tritium released by electrical stimulation is sampled using a suitable pipette.