Hypoxia induces voltage-dependent Ca2+ entry and quantal dopamine secretion in carotid body glomus cells

We have investigated the changes of cytosolic [Ca2+1 and the secretory -activity in single glomus cells dispersed from rabbit carotid bodies during exposure to solutions with variable 02 tension (Po2). In normoxic conditions (Po2= 145 mmHg; 1 mmHg = 133 Pa), intracellular [Ca2+J was 58 1 29 nM, and switching to lo'iV P02 (between 10 and 60 mmHg) led to a reversible of [Ca2+1 up to 800 nM. The response to hypoxia completely disappeared after removal of external Ca2+ or with the addition of 0.2 mM Cd2+ to the external solution. These same solutions also abolished both the Ca2+ current of the cells and the increase of internal (Ca2+ elicited by hig external K+. Elevations of cytosolic [Ca2e+ in response to hypoxia or to direct membrane depolarization elicited the release of dopamine, which was detected by amperometric techniques. Dopamine secretion occurred in episodes of spike-like activity that appear to represent the release from single secretory vesicles. From the mean charge of well-resolved secretory events, we estimated the average number of dopamine molecules per vesicle to be =140,000, a value about 15 times smaller than a previous estimate in chromaffin granules of adrenomedullary cells. These results directly demonstrate in a single-cell preparation the secretory response of glomus cells to hypoxia. The data indicate that the enhancement of cellular excitability upon exposure to low Po2 results in Ca2+ entry through voltage-gated channels, which leads to an increase in intracellular [Ca2+J and exocytotic transmitter release