Calcium channels physiological role

One of the characteristics of protein calcium channels is their sensitivity to ablock by transition metal cations. Lanthanum is a particularly potent blocker. It is suggested that permeant and blocking ions compete for the common binding sites in the channels. The PolyP-PHB channel complexes are also blocked by transition metal cations in a concentration-dependent manner. A nearly complete block of single-channel currents was observed in the synthetic complexes at concentations > 0.1 mM La3+ (0.1 % of Ca2+) (Das et al., 1997). Evidently, PHB-PolyP complexes are versatile ion carriers whose selectivities may be modulated by small adjustments of the local pH. The results may be relevant to the physiological function of PHB-PolyP channels in bacteria and the role of PHBs and PolyPs in the Streptomyces lividans potassium channel (Das and Reusch, 2001). 

The physiological role of PHB/polyP complexes has not been established, but bacterial cells like eukaryotic cells maintain low internal Ca2+,101-103,112 and there is increasing evidence pointing to calcium involvement in a number of important cellular functions, such as chemotaxis, cell division, heat shock, pathogenicity, and differentiation.104-107 Systems for calcium export have been identified.108-109 Mechanisms for calcium entry are less well known. An L-type channel was reported in Bacillus subtilis,n0 lu and Jones et al. and Holland et al.103,112 recently demonstrated the presence of a putative Ca2+ influx channel in stationary phase E. coli,... 

Sequence homology within conotoxin subtypes appears not to be related to receptor selectivity. For example, the cj-conotoxins found in several piscivorous cone snail species all paralyze fish and target the presynaptic calcium channels, yet they have up to 70% difference in their sequences. The only amino acids that are conserved are the Cys residues that provide the basic framework of these particular conotoxins. These findings indicate that the non-Cys amino acids are crucial in the physiological role of conotoxins. The King-Kong peptide (named from the aggressive response that it causes in lobsters) isolated from Conus textik has retained the... 

Besides their therapeutic applications, conotoxins have exhibited great potential for the development of neurological probes. Conotoxins are currently being used in hundreds of research laboratories for a wide variety of physiological investigations. Some conotoxins have become well-established neurobiological tools. They can also be used as antagonists of specific subtypes in functional studies for example, w-conotoxin MVIIA is used as a specific N-type calcium channel blocker. Conotoxins are also utilized in research to provide information on the role and distribution of different receptor subtypes. Undoubtedly, these research applications will continue to expand. 

This chapter will examine the properties (structure, distribution), pharmacology (openers and blockers as tools to study K+ channels) of calcium and potassium channels. Emphasis will be on integration of the information to develop a picture of the physiological roles of the different types of ion channels. Since potassium channels regulate arterial smooth muscle function by controlling membrane potential, the membrane potential and its relationship to smooth muscle function will be discussed first, as well as the role that K" channels play in controlling membrane potential. 

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