Modulation of kir4.1 and kir5.1 by hypercapnia and intracellular acidosis.

CO2 central chemoreceptors play an important role in autonomic respiration. An increase in PCO2 level may augment the tonic activity of these chemoreceptors that project to the respiratory central pattern generators, leading to an increase in respiratory motor output (Mitchell & Berger, 1975; von Euler, 1986). These chemoreceptors were firstly demonstrated in the ventralateral medulla (Mitchell et al. 1963; Schlaefke et al. 1970; Loeschcke, 1973) and later found in several other brainstem nuclei (Schlaefke et al. 1979; Dean et al. 1989, 1990; Nattie et al. 1993; Richerson, 1995; Kawai et al. 1996; Pineda & Aghajanian, 1997; Oyamada et al. 1998; Wang et al. 1998; Wellner-Kienitz et al. 1998). The effect of CO2 on neurons in brainstem chemoreceptive areas exists following the blockade of synaptic transmission, indicating that the effect is postsynaptic (Dean et al. 1990; Kawai et al. 1996; Oyamada et al. 1998). It is possible that CO2 sensing in these neurons is conducted by certain molecules that are CO2 sensitive and couple the change in PCO2 levels to a change in membrane excitability. The inward rectifier K+ (Kir) channel appears to be one of these potential CO2 sensing molecules. Certain genetically unidentified Kir channels have been shown to be inhibited by pH and CO2 around physiological levels in brainstem neurons and renal epithelial cells (Schlatter et al. 1994; Zhou & Wingo, 1994; Pineda & Aghajanian, 1997). It is known that these K+ channels are responsible for the maintenance of membrane potential. In fact, there is experimental evidence indicating that inhibition of Kir channels produces depolarization (Pineda & Aghajanian, 1997). Molecular structures and biophysical properties of numerous Kir channels have been well studied in molecular clonings over the past 5 years. Some of the cloned Kir channels show a similar CO2/pH sensitivity to those in brainstem neurons (Coulter et al. 1995; Tsai et al. 1995; Doi et al. 1996; Qu et al. 1999). Although these observations suggest the involvement of Kir channels in hypercapnia, their specific expression has not been confirmed in the brainstem. Therefore, it is necessary to know which members of the Kir family that are expressed predominantly in the brainstem are CO2/pH sensitive. The Kir4.1 channel is a member of the Kir family which is known to be mainly expressed in the brainstem (Bredt et al. 1995). This channel is inhibited by ATP (Bredt et al. 1995), and is proton sensitive (Yang & Jiang, 1999). Two other Kir channels with a high sequence similarity to brain Kir4.1 (i.e. Kir1.2 and Kir4.2 cloned from the kidney and liver, respectively) are also inhibited by low pH (Shuck et al. 1997; Pearson et al. 1999). However, the pH sensitivity of these Kir4 channels is low (pK 6.0–6.2) which may go against their participation in CO2 detection (Shuck et al. 1997; Yang & Jiang, 1999). Brain Kir4.1 is coexpressed with brain Kir5.1, a subunit which does not produce a functional channel as a homomultimer (Pessia et al. 1996). Kir5.1, however, has a molecular motif identical to a critical sequence in pH sensing of Kir2.3 (Qu et al. 1999), which may provide the heteromeric K+ channels with a pH sensing mechanism in addition to those existing in Kir4.1. Lysine 67 in Kir4.1 may be involved in the pH sensing mechanisms since a corresponding residue (Lys80) in Kir1.1 plays a critical part in the channel sensitivity to pH (Fakler et al. 1996). To test these possibilities, we performed experiments in which brain Kir4.1 was coexpressed with brain Kir5.1, and CO2 and pH sensitivity of the heteromeric K+ channels were studied.