Spontaneously beating myocardial cells isolated from the hearts of chick embryos aged 2-18 days were cultured in medium containing 1.3, 4.2, 8.1, or 12 mM K+. At all ages tested, the percentage of beating cells (% BC) was maximal at 1.3 mM K. In cultures from 2-to 4-day hearts, isolated pacemaker cells were relatively insensitive to the level of extracellular K-concentration (K0). In contrast, the activity of most of the cells from 7- to 18-day hearts was suppressed by high Ko. The first week of heart development, therefore, represents a period of transition from a state of K-insensitivity to one of greater sensitivity of cardiac pacemakers to the level of Ko. Differences in the kinetics of pacemaker-inhibition in response to a sudden increment of Kowere used as evidence for three subpopulations of spontaneously active cells: those in which spontaneous activity was immediately and permanently suppressed; those which stopped beating initially but gradually recovered; and those which were unaffected by an increment in Ko. The relative size of each of these subpopulations was found to change in a systematic way with embryonic age. It is suggested that the change from a state of K-resistance to K-sensitivity of the embryonic pacemakers results either from a change in ionic permselectivity of the pacemaker membranes, or from an alteration in the concentration of ions inside and outside the cells during early development. © 1970.
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