NONGENOMIC EFFECT OF TRIIODOTHYRONINE ON CELL-SURFACE BETA-ADRENOCEPTORS IN CULTURED EMBRYONIC CARDIAC MYOCYTES

We studied the time course of cell surface beta-adrenoceptors (BAR) in cardiomyocytes in response to a single triiodothyronine (T-3) (10(-8) M) stimulation. An early first increase of BAR density was observed w ithin 2 hr (+10% versus control cells, p < 0.05), and a plateau was ma intained for 17-20 hr. This effect was followed by a much greater, lat e increase of BAR density, starting around 22 hr and lasting until 48 hr post T-3 addition (+40% versus control cells; p < 0.05). Since reve rse T-3 studied in the same conditions had no effect in this system, w e concluded that this T-3 effect was specific. We hypothesized that th e early response might be nongenomic because the early effect of T-3 w as still observed in the presence of cycloheximide (2 x 10(-5) M) wher eas the late increase was totally suppressed by the drug. The early re sponse to T-3 required intact microtubules, since colchichine (2 x 10( -5) M) was able to block the increase in the cell surface BAR number, but it did not involve a change in BAR distribution between external a nd internal sites, as the external to total BAR ratio remained stable. The measurement of the rate of BAR disappearance from the cell surfac e allowed us to hypothesize that T-3 induced a modification of the tur nover in cell surface BAR.