Ionic currents in carotid body type I cells isolated from normoxic and chronically hypoxic adult rats

Whole-cell recordings were used to investigate the effects of a 3-week period of hypoxia (10% O₂) on the properties of K⁺ and Ca²⁺ currents in type I cells isolated from adult rat carotid bodies. Chronic hypoxia significantly increased whole-cell membrane capacitance. K⁺ current amplitudes were not affected by this period of hypoxia, but K⁺ current density was significantly reduced in cells from chronically hypoxic rats as compared with normoxically maintained, age-matched controls. K⁺ current density was separated into Ca²⁺-dependent and Ca²⁺-independent components by bath application of 200 μM Cd²⁺, which blocked Ca²⁺ currents and therefore, indirectly, Ca²⁺-dependent K⁺ currents. Ca²⁺-dependent K⁺ current density was not significantly different in control and chronically hypoxic type I cells. Cd²⁺-resistant (Ca²⁺-insensitive) K⁺ current densities were significantly reduced in type I cells from chronically hypoxic rats. Acute hypoxia (Po₂ 15–22 mmHg) caused reversible, selective inhibition of Ca²⁺-dependent K⁺ currents in both groups of cells and Ca²⁺-insensitive K⁺ currents were unaffected by acute hypoxia. Ca²⁺ channel current density was not significantly affected by chronic hypoxia, nor was the degree of Ca²⁺ channel current inhibition caused by nifedipine (5 μM). Acute hypoxia did not affect Ca²⁺ channel currents in either group. Our results indicate that adult rat type I cells undergo a selective suppression of Ca²⁺-insensitive, voltage-gated K⁺ currents in response to chronic hypoxia in vivo. These findings are discussed in relation to the known adaptations of the intact carotid body to chronic hypoxia.     

Inhibition of Toosendanin on the delayed rectifier potassium current in neuroblastoma×glioma NG108-15 cells

The effect of Toosendanin (TSN), a presynaptic transmission blocker, on the outward delayed rectifier potassium current (I_KD) of NG108-15 cells was studied by using the whole-cell voltage-clamp technique. It was observed that externally applying TSN not only reduced I_KD amplitude in a dose-dependent and partial reversible manner but also accelerated its inactivation. The effect of internally applying TSN was also examined by including TSN in the electrode, and it was the same as that of externally applying TSN. Further, comparison observations with TEA, 4-AP, verapamil, nifedipine, and (±)-Bay K 8644 were also made, and the results were as follows. The time courses of TSN's inhibition effect as well as its recovery after washing were much slower than those of TEA and 4-AP. Externally applying TEA or 4-AP reduced I_KD amplitude but did not accelerate its inactivation. Externally applying verapamil, nifedipine, or (±)-Bay K 8644, however, similarly to the effect of TSN, not only reduced I_KD amplitude but also accelerated its inactivation. Thus, from the obtained results it is suggested that TSN might diffuse into the cell interior and act intracellularly, and the underlying mechanism might be different from that of TEA and 4-AP but similar to that of verapamil, nifedipine, and (±)-Bay K 8644 to some extent. © 1997 Elsevier Science B.V. All rights reserved.     

Arachidonic acid inhibits both K and Ca currents in isolated type I cells of the rat carotid body

Whole-cell patch-clamp recordings were used to investigate the effects of arachidonic acid (AA) on K⁺ and Ca²⁺ channels in isolated rat type I carotid body cells. AA (2–20 μM) produced a concentration-dependent inhibition of both K⁺ currents and Ca²⁺ channel currents. The effects of AA on K⁺ currents were unaffected by indomethacin (5 μM), phenidone (5 μM) or 1-aminobenzotriazole (3 mM), suggesting that AA did not exert its effects via cyclo-oxygenase, lipoxygenase or cytochrome P-450 (cP-450) metabolism. Our results suggest that AA directly and non-selectively inhibits ionic currents in rat type I carotid body cells.     

Effects of doxapram on ionic currents recorded in isolated type I cells of the neonatal rat carotid body

Whole-cell patch-clamp recordings were used to investigate the effects of the respiratory stimulant doxapram on K+ and Ca2+ currents in isolated type I cells of the neonatal rat carotid body. Doxapram (1-100 microM) caused rapid, reversible and dose-dependent inhibitions of K+ currents recorded in type I cells (IC50 approximately 13 microM). Inhibition was voltage-dependent, in that the effects of doxapram were maximal at test potentials where a shoulder in the current-voltage relationship was maximal. These K+ currents were composed of both Ca(2+)-activated and Ca(2+)-independent components. Using high [Mg2+], low [Ca2+] solutions to inhibit Ca(2+)-activated K+ currents, doxapram was also seen to directly inhibit Ca(2+)-independent K+ currents. This effect was voltage-independent and was less potent (IC50 approximately 20 microM) than under control conditions, suggesting that doxapram was a more potent inhibitor of the Ca(2+)-activated K+ currents recorded under control conditions. Doxapram (10 microM) was without effect on L-type Ca2+ channel currents recorded under conditions where K+ channel activity was minimized and was also without significant effect on K+ currents recorded in the neuronal cell line NG-108 15, suggesting a selective effect on carotid body type I cells. The effects of doxapram on type I cells show similarities to those of the physiological stimuli of the carotid body, suggesting that doxapram may share a similar mechanism of action in stimulating the intact organ.