Electrical properties of chemoreceptor cells

66 multiple types of K· channels. Below, we consider the separa te ionic currents of type I cells, and compare results obtained from different preparations in different laborato ries. NA+ CURRENTS Using standard procedures for blocking K· currents (i.e., intracellular Cs+) and ea2+ currents (i.e., extracellular Cd2+), Na+ cur rents (INa) can be routinely recorded in iso lation in adult rabbit type I cellsl.4 (Fig. 4. lA). INa is sensitive to tetrodotoxin (TTX), has an apparent activation threshold around -40 m V and a fast activation time course, reaé:hing its peak in less than 1 ms. The peak amplitude at O m V is approximately 0.4 nA on average. The half steady-state inactiva tion occurs at a membrane potential of -50 A �-ª 0.2 .... L s .. B e The Carotid Body Chemoreceptors m V. Inactivation follows a monoexponential time course, with a time constant of 0.67 ms at O m V. All these characteristics are typical of Na• currents found in other neurosecre tory cells. Na+ currents have not been found in rab bit embryos5 and their presence is not ubiq uitously reported in rat type I cells. Sorne groups have reported a lack ofNa+·currents in freshly dissociated type I cells from adult6 and young rats.7 However, other groups have reported rapidly activating and inactivating TTX-sensitive Na+ currents present in cells from young rats maintained in long-term culturesª or in freshly dissociated cells from adult9 and neonatal rats.10 The wide range of reported percentages of cells having Na+ currents in rat cells (from noné7 to 46% 10) may reflect age-related differences or more u�. ·- VliNl(mV) �..0-200204090 l(Ml Fig. 4.1. Whole-cell inward currents recorded in adult rabbit (A & B) and neonatal rat (C) cells. (A) Family of Na• currents recorded in the presence of 0.1 mM Cd2+. The 1-V relationship of the peak current is also shown. (B) 1-V relationship for Ca2+ currents obtained in an adult rabbit chemoreceptor cell using 1 O mM Ca2• (filled circles) or 1 O mM Ba2• (open circles) as charge carriers. Na• currents are blocked with 0.1 mM TTX. The currents obtained with Ca2• and Ba2• at + 1 O mV are shown superimposed. (C) Ca2+ channel currents recorded in a neonatal rat type I cell using 1 O mM Ba2+ as charge carrier. Note the lack of rapidly activating and inactivating Na• current, despite the fact that TTX was not present. Ca2• channel currents could be fully blocked by 100 mM Cd2+ and enhanced by the DHP agonist Bay K 8644 (S mM). The 1-V relationship from the same cell is shown in the ab sence (filled circles) and presence (open circles) of Bay K 8644. Gonzalez-lext 66 5/29197, 3:58 PM