Nonselective Cation Channels

Two types of nonselective cation channels have been reported to be present in smooth muscle cells; a hyper-polarization-activated nonselective cation channel and a receptor-operated nonselective cation channel. Hy-perpolarization-activated channels have been recorded in cardiac muscle, neurons, and other excitable cells, and this channel is thought to contribute to pacemaker activity and to the quick recovery from the afterhyper-polarization (which permits the generation of a train of action potentials at a high frequency) (DiFrancesco, 1995; Pape, 1996). In smooth muscle cells, this current is also observed in cells that exhibit spontaneous electrical activity, such as those in the small intestine, uterus, bladder, and portal vein (Benham etal., 1987; Kamouchi et al., 1991; Green et al., 1996; Okabe et al., 1999). However, such channels are not present in vascular cells, except in the portal vein.

The other type of nonselective cation channel is activated by receptor stimulation. This type of channel has been further classified into two types: ligand-gated and G-protein-coupled nonselective cation channels (Kuriy-ama et al., 1998). The P2X1 channel is an example of an ion channel activated by direct binding of the ATP molecule to the channel protein; this channel is present in vascular cells. This channel is activated by ATP and by several of its analogues, and the current is inactivated rapidly (Benham et al., 1987a; Benham and Tsien, 1988; Benham, 1989; Xiong et al., 1991). This channel permeates several cations, including Na+, K+, and Ca2+. The permeability ratio for Na+:Ca2+ has been reported to be 1:3-4 in various cells (Benham and Tsien, 1988; Honore et al., 1989). The P2X1 channel is thought to be a homotrimer of subunits with two transmembrane domains (Valera et al., 1994; Nicke et al., 1998). Although heteromultimer structures have been proposed for other P2X receptors, P2X1 mRNA signals have been predominantly observed in vascular cells, indicating the high likelihood of the presence of a homomultimer conformation (Soto et al., 1996; Garcia-Guzman et al., 1996; Le et al., 1997; Longhurst et al., 1996; Bo et al., 1998; Torres et al., 1998,1999). The presence of a weak mRNA signal for a splice variant P2X1 subunit in rat cerebral and mesenteric arteries, as well as in cardiac and vas deferens smooth muscle cells, has been shown (Ohkubo et al., 1999). Thus, a heteromultimer conformation involving wild and variant subtypes may be present in vascular cells. It has been reported that the y phosphate of ATP interacts with the P-loop region located near the second transmembrane domain (Hansen et al., 1997). The P2X1 receptor shows a transient current with rapid inactivation, and for this channel property both transmembrane regions are essential; however, the P2X2 receptor channel, which is a noninactivating channel, acquires its inactivating property by C-terminal

FIGURE 8 PDBu/GTP-induced nonselective cation currents in rabbit portal vein. Three micromolar GTPyS was added to the pipette solution (Cs+ aspartate), and 5 mM Ba2+-containing physiological salt solution was superfused in the bath. PDBu (0.3 ^M) was applied to the bath. At (a) and (b), a ramp pulse (from -100 to 50 mV) was applied from the holding potential of -60 mV. Obtained currents by ramp pulses are demonstrated in B. (C) Relationships of the amplitude of PDBu/GTP-induced current and PDBu concentrations in the absence and presence of 3 ^M GTPyS. The PDBu/GTP-induced current was predominantly recorded in the presence of GTPyS. (M. Oike and K. Kitamura, unpublished observations)

PDBu concentration QjM)

FIGURE 8 PDBu/GTP-induced nonselective cation currents in rabbit portal vein. Three micromolar GTPyS was added to the pipette solution (Cs+ aspartate), and 5 mM Ba2+-containing physiological salt solution was superfused in the bath. PDBu (0.3 ^M) was applied to the bath. At (a) and (b), a ramp pulse (from -100 to 50 mV) was applied from the holding potential of -60 mV. Obtained currents by ramp pulses are demonstrated in B. (C) Relationships of the amplitude of PDBu/GTP-induced current and PDBu concentrations in the absence and presence of 3 ^M GTPyS. The PDBu/GTP-induced current was predominantly recorded in the presence of GTPyS. (M. Oike and K. Kitamura, unpublished observations)

truncation (Werner et al., 1996; Brandle et al., 1999). Therefore, the absence of a long C-terminal in the P2X1 subunit is also important for its inactivating properties, as is the presence of a P2X1-type transmembrane motif.

The G-protein-coupled purinoceptor also activates a nonselective cation channel in the rabbit portal vein (Xiong et al., 1991). This current did not inactivate, and higher concentrations of ATP (submicromolar order) were required for channel activation, whereas a^-meth-ylene ATP had only a very weak action. In contrast to the P2X1 receptor current, these authors also showed that the purinoceptor-mediated current was coupled to a pertussis toxin-insensitive G-protein and did not in practice permeate Ca2+ ions (Xiong et al., 1991). G-protein-coupled nonselective cation channels are also activated by stimulations of other receptors, which are generally coupled with Gqa proteins (a adrenoceptor in portal vein: Byrne and Large, 1988; Inoue and Kuriyama 1993; Yamada et al., 1996; endothelin in mesenteric artery: Chen and Wagoner, 1991; Enoki et al., 1995). Ionic properties similar to those of the pertussis toxin-insensitive current were also seen with GTP and PDBu when these were applied simultaneously (Oike et al., 1993). Therefore, it is likely that simultaneous modulation of the channel, through direct G-protein binding and PKC phosphorylation, is essential for its activation by receptor stimulation (Fig. 8).

''window current'' could be evoked. It is uncertain whether Ca2+ and Na+ influx through a window current is enough to elevate the [Ca2+j; and membrane depolarization, as reevaluation of the channel properties under the phsiological condition is necessary for elucidation of their physiological functions.

Patch clamp experiments revealed various types of ion channels in vascular smooth muscle cells, and differences of electrical properties of vascular cells from visceral and cardiac cells might be caused by differences of distribution and density of each type of ion channel. The presence of nonselective cation channels activated by agonist binding to the receptors is especially important for direct and secondary activation of the voltage-dependent ion channels, such as voltage-dependent Ca channels, and could specify the electrical activities of the vascular smooth muscle cells. However, it has been reported that first-order structures of the ion channels in vascular smooth muscle cells are not identical to the corresponding channels in other tissues. As there were very few reports showing that single or few points mutation of amino acid in the channel protein changed their pharmacological and physiological properties, speculation of channel properties from other known tissues was limited. Thus, further evidence with molecular biological experiments in vascular cells will be required for evaluating ion channels in vascular cells.

Was this article helpful?

0 0

Post a comment