Acetylcholine and electrolyte metabolism in the various chambers of the frog and turtle heart

EFFECT OF CURRENT ON TRANSMEMBRANE POTENTIALS IN CULTURED CHICK HEART CELLS

By means of a DC bridge circuit one microelectrode was used for simultaneously passing current and recording transmembrane potentials. In some cells, depolarization increased the frequency of discharge whereas hyperpolarization decreased the frequency; the frequency/current relation was sigmoid. In other cells, polarizing currents were without effect upon frequency. The change in action potential magnitude was in proportion to the degree of polarization. From control values of about 5 mv/sec., the slope of the pacemaker potential increased to 60 mv/sec. upon depolarization and diminished to zero upon hyperpolarization. In many cells a transient hyperpolarization was produced on the cessation of depolarizing currents. The voltage/current relationship was linear and had a slope of about 13 MΩ. With an AC bridge circuit, the cell capacitance averaged 800 pf and the time constant, 9.6 msec. R_m was estimated to be 480 Ω-cm² and C_m, 20 µf/cm². The magnitudes of some prepotentials were affected by polarizing currents, which suggests that the prepotentials represent postsynaptic potentials.

Choline acetyltransferase in the heart of adult rats

The distribution of choline acetyltransferase (ChAT, EC 2.3.1.6.) in the heart of adult rats has been reinvestigated in view of recent discoveries that acetylcholine (ACh) can be synthesized not only by ChAT, but also by carnitine acetyltransferase (CarAT, EC 2.3.1.7) and that it is possible to distinquish between the ACh-synthesizing activity of ChAT in intramuscular nerves and the CarAT-mediated extraneural synthesis of ACh by means of bromoacetylcholine (BrACh), a specific inhibitor of ChAT. BrACh (0.002 mmol/l) has been found to inhibit the synthesis of ACh in the atria by 66-85% and in the ventricles by only 19-29%. Bromoacetylcarnitine (BrACar, 0.02 mmol/l), and inhibitor of CarAT, inhibited the synthesis of ACh in the atria by 34% and in the ventricles by 74-80%. These findings indicate that ChAT is responsible for most of the synthesis of ACh observed in the homogenates of the atria; in the ventricles, it catalyses only a minor portion of the total ACh synthesis observed. In the investigation of the regional distribution of ChAT in the heart, the BrACh-sensitive part of ACh synthesis was taken as the measure of ChAT activity. The highest activity of ChAT (nmol ACh synthesized g-l.h-l) was found in the region of the sinoatrial node (1775); it decreased in the order: interatrial septum (781) greater than rest of the right atrium (712) greater than left atrium (416) greater than basal part of the right ventricle (366) greater than apical part of the right ventricle (250) greater than inter-ventricular septum (239) greater than basal and apical part of the left ventricle (208 and 205). The results indicate that earlier investigations of the distribution of ChAT in the heart provided a basically correct picture although the contribution of CarAT to the synthesis of ACh measured had not been excluded, and confirm that ChAT is present throughout the heart, including the apical parts of the ventricles, However, the sino-atrio-ventricular gradient of ChAT distribution is steeper when the contribution of CarAT to the synthesis of ACh is excluded.

The effects of external sodium and potassium concentration on the membrane potential of atrioventricular fibers of the toad

Intracellular records were made from fibers in the A-V conducting system of isolated toad hearts. The A-V region was perfused with Ringer's solution of various K and Na concentrations. Resting potential in 2.8 mM [K]o was about 60 mv. Over the range 0.28 to 28 mM, resting potential diminished with increasing [K]o. Spontaneous action potentials appeared when [K]o was increased to 11.2 mM, and when resting potential had fallen to about 40 to 50 mv. Changes in [Na]o over the range 22 to 110 mM had a little effect on resting potential, but there was a linear relation between the peak value of the action potential and log [Na]o. Wenckebach periodicity was observed when [Na]o was lowered.