Maximum Upstroke Velocity
Ordinary Myocardial Muscle Fibers
Springer Online Journal Archives 1860-2000
Summary 1. The transmembrane resting and action potentials of single fibers from the ventricle and from the specialized conducting system of the hearts of guineapigs, cats and rhesus monkeys have been studied by means of intracellular microelectrodes. Circuits were included which ensured capacity compensation of the whole microelectrode assembly and, further, allowed measurement of the maximum upstroke velocity (MUV) by analogue differentiation. 2. The MUV in Purkinje fibers was around 400 V/s. As the ventricular wall was approached there was a great fall in the MUV. In the intervening transition area the MUV lay between 200 and 300 V/s. In ordinary myocardial muscle fibers the MUV was within the range 80–200 V/s (average value of 526 observations, 165.4±1.7 V/s, at a resting potential of −74.1±1.3 mV; ±S.E.s of means). 3. Whereas the MUV in an ordinary muscle fiber was unrelated to the individual resting potential, a depolarizing current depressed the MUV in parallel with the diminution of the membrane potential. However, the steepness of the MUV-membrane potential relation was low in ordinary muscle fibers compared with that in Purkinje fibers. 4. With respect to Ca the three tissues (Purkinje-, transition- and ordinary muscle fiber) exhibited different sensitivities. A 3–4 fold increase in extracellular Ca concentration diminished the MUV in ordinary muscle fibers (in spite of a raised resting potential and overshoot), but raised the MUV always in Purkinje fibers and often in the transition area cells. 5. Upon changing the topographical recording sites progressively from Purkinje fibers through transitional to ordinary cardiac muscle fibers spontaneous diastolic depolarization was no longer observed and there was a decrease in the MUV and conduction velocities. In addition, the Purkinje and ordinary muscle fibers showed different sensitivities of the rising phase of the action potential both to alterations in the membrane potential and to ionic influences.
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