Chronotropic responses of human heart tissue cultures.

Three presumptive cardiac tissues from aborted human embryos which ranged in length from 2 mm to 4 mm and in age from 53 days to 71 days were maintained in culture beating rhythmically for 18-88 days. Medium renewal, temperature increases (from 31° to 38°C), and pH increases (from 6.92 to 7.75) raised the pulsation rate. Excess potassium stopped the pulsation. Sympathomimetic drugs increased the pulsation rate, and this action was blocked by the beta-receptor inhibitor, propranolol, suggesting the presence of functioning beta-receptor sites in the heart cells by the end of the second month of development.