Introduction

Acetylcholine (ACh) acts on muscarinic receptors to cause excitation of a wide range of smooth muscles. Early studies on multicellular smooth muscle preparations led to the view that muscarinic stimulation caused depolarization by activating channels permeable to Na+, K+, and Ca2+ (Bolton 1981). This proposed action of ACh in smooth muscle was similar to that described for neuromuscular transmission in skeletal muscle (Takeuchi and Takeuchi 1960; Katz 1966), where ACh binds to nicotinic ACh receptors which also function as nonselective cation channels. Support for this proposal was elusive in early studies of smooth muscles because it was difficult to apply the voltage-clamp technique – the method of choice for determining membrane conductance changes – to multicellular preparations, in large part because of difficulties in obtaining adequate spatial and temporal control of membrane potential (see the references in Singer and Walsh 1980a; or in Bolton et al. 1981). For this reason, the primary effect of ACh on membrane channels could not be distinguished from secondary changes accompanying depolarization. For example, ACh causes depolarization which in turn activates voltage-dependent Ca2+ channels (see Figure 1). Without voltage clamp, it would be impossible to distinguish between ACh activating Ca2+ channels directly or through an indirect mechanism involving depolarization.

Several important breakthroughs permitted clear and decisive studies of voltage- and transmitter-activated channels in smooth muscle. The first was the development of techniques for dissociating smooth muscle tissues into single cells suitable for physiological studies (Bagby et al. 1971; Fay and Delise 1973; reviewed in Sanders 1989).