Risk and advantages of using strongly beveled microelectrodes for electrophysiological studies in cardiac Purkinje fibers.

Conventional microelectrodes (tips with a diameter of 0.5 micrometer, a resistance of 8 MOhm, and a tip potential of -4 mV) were mechanically beveled over a length of 1--2 micrometer (resistance 2.5 MOhm, tip potential between 0 and -1 mV). Properties thought to be relevant for intracellular techniques were studied on the cardiac Purkinje fiber. The comparison with conventional microelectrodes suggests that beveled microelectrodes have an advantage as stated: 1. Intracellular impalement is favoured by the proper shape and the greater mechanical stability; the beveled tip penetrates the connective tissue smoothly without breaking or plugging. 2. Current injection (constant current mode) can be done without blocking or polarizing the tip. The 2 microelectrode voltage clamp technique (Deck et al., 1964) is improved by having lower noise and better stability, by a faster response time, and a greater range of clamp potentials (up to +80 mV). 3. The spontaneous release of the electrolytes filling the microelectrode has a 3-fold greater rate (0.015 pMol/s); this enlargement does not change the electrophysiological properties of the fiber. 4. Intracellular pressure injection requires pressures of 0.3--1.5 bar only to inject the solutes with rates between 1 and 100 pMol/s.