Thyrotropin-releasing hormone (TRH) causes a transient hyperpolarization followed by several minutes of increased action potential frequency in patch-perforated current-clamped GH3 cells. Treatment of cells for 5 min with either 2 or 100 nM of the protein phosphatase inhibitor okadaic acid does not affect electrical activity of the cells, but potentiates the enhancement of action potential frequency elicited by a subsequent addition of TRH. Alternatively, 100 nM (but not 2 nM) of okadaic acid added during the second phase of TRH action, further increases the frequency of firing above that produced by the hormone. Similar effects to those of 2 nM okadaic acid are observed with 20 nM calyculin A. These data suggest that a protein phosphatase plays a major role in regulating the delayed effects of TRH on cell excitability in GH3 cells.
