Effector mechanisms for α,β-methylene ATP and ATP derivatives in guinea-pig taenia caeci

Abstract

The effects of the ATP analog α,β-methylene ATP and ATP derivatives on smooth muscle cells of guinea-pig taenia caeci were studied by measuring potential changes. The preparations were superfused (1 ml/min) with Krebs solution or calcium-free solution containing atropine (10 −6 M), prazosin (10 −6 M) and propanolol (10 −6 M) using the sucrose-gap method at 22°C. The ATP analog (10 −5 M to 4 × 10 −4 M) caused slowly developing hyperpolarization in the presence of calcium. Hyperpolarization also occurred on addition of the analog in calcium-free medium. The area of the response increased with the analog concentration (10 −5 M to 4 × 10 −4 M). The response evoked after repeated addition of the ATP analog had slightly lower amplitude and slower onset than the first response both in the presence and the absence of calcium. The area of the response to the ATP analog in the absence of calcium was also decreased when preceded by the ATP response. When the opposite experimental sequence was used the ATP response was strongly inhibited, when preceded by the analog response evoked by the same concentration. The ATP analog caused depolarization of the muscle cells in the presence of apamin (3 × 10 −7 M), which was also observed with ATP. The ATP derivatives (4 × 10 −4 M) caused hyperpolarization of the smooth muscle cell membrane, except hypoxanthine and inosine which were inactive. The responses evoked by the nucleotides were characterized by hyperpolarization reaching a maximum followed by a sustained component in calcium-free conditions, the latter being more pronounced with AMP than with ATP. The sustained hyperpolarization persisted during the second response or after a preceding ATP response. In the presence of apamin, initial depolarization was followed by sustained hyperpolarization. The response to adenosine was reflected by sustained hyperpolarization in both the absence and presence of calcium or apamin. The inhibitory junction potential elicited after electrical stimulation (2 s, 5 pps) of taenia caeci and the response evoked by exogeneously applied ATP, both reflected by hyperpolarization of the muscle cells, reached the same level in the presence or absence of α,β-methylene ATP. The results indicate that α,β-methylene ATP opens apamin-sensitive potassium channels in smooth muscle cells of taenia caeci directly and via a mechanism also activated by ATP and the ATP nucleotides ADP and AMP.