Thyroid follicular cells: the resting membrane potential and the communication network.

Intracellular recordings of membrane potentials, input resistance and time constant have been made in vitro from the follicular cells of the rat, rabbit and guinea-pig thyroid glands using glass microelectrodes. The passive permeability properties of these cells have been investigated by altering the concentration of one or more ions in the superfusing fluid. Investigations into the intercellular coupling characteristics of the thyroid gland were made by inserting two microelectrodes into neighbouring communicating cells. The mean transmembrane potentials were between - 60 and - 70 mV in all three species studied. The magnitude of the membrane potential in the rat was found to be dependent mainly upon the gradient for potassium (K+) across the membrane. Current-voltage relationships were investigated in all three species by injecting rectangular de- or hyperpolarizing current pulses through the recording microelectrode. Within a relatively wide range (- 20 to - 80 mV), there was an approximately linear relationship between injected current and change in membrane potential. The input resistance was about 11 Momega in all three species, while the time constant (tau) varied from 5-35 ms. Readmitting K to K-deprived rat thyroids during intracellular microelectrode recording caused a transient hyperpolarization which was unaccompanied by any change in input resistance. The transient hyperpolarization was abolished by ouabain. Addition of 10(-3) M ouabain to the resting cell caused an immediate depolarization of approximately 2 mV. Electrical coupling between neighbouring cells could only be observed if the distance between the tips of the two exploring microelectrodes was less than 15 micrometer. The coupling coefficient (V2/V1) was close to 1. Assuming uniform current spread within one follicle and electrical isolation of individual follicles from each other the specific membrane resistance of the rat thyroid follicular cells was calculated to be 4.9 komegacm2.