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Interaction of tolbutamide and cytosolic nucleotides in controlling the ATP‐sensitive K + channel in mouse β‐cells
Author(s) -
Schwanstecher Christina,
Dickel Corinna,
Panten U.
Publication year - 1994
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1994.tb14060.x
Subject(s) - tolbutamide , gtp' , guanosine , adenosine triphosphate , nucleotide , biochemistry , chemistry , guanosine triphosphate , adenosine , adenosine diphosphate , adenine nucleotide , cytosol , uridine triphosphate , biology , enzyme , endocrinology , insulin , platelet , platelet aggregation , gene , immunology
1 In mouse pancreatic β‐cells the role of cytosolic nucleotides in the regulation of the sulphonylurea sensitivity of the adenosine 5′‐triphosphate‐sensitive K + channel (K ATP ‐channel) was examined. Patch‐clamp experiments with excised inside‐out membrane patches were carried out using an experimental protocol favouring phosphorylation of membrane proteins. 2 In the absence of Mg 2+ , the K ATP ‐channel‐inhibiting potency of cytosolic nucleotides decreased in the order ATP = adenosine 5′‐O‐(3‐thiotriphosphate) (ATPγS) > adenosine 5′‐diphosphate (ADP) > adenosine 5′‐O‐(2‐thiodiphosphate) (ADPβS) = adenylyl‐imidodiphosphate (AMP‐PNP) > 2′‐deoxyadenosine 5′‐triphosphate (dATP) > uridine 5′‐triphosphate (UTP) > 2′‐deoxyadenosine 5′‐diphosphate (dADP) > guanosine 5′‐triphosphate (GTP) > guanosine 5′‐diphosphate (GDP) > uridine 5′‐diphosphate (UDP). 3 In the presence of Mg 2+ , the inhibitory potency of cytosolic nucleotides decreased in the order ATPγS > ATP > AMP‐PNP > ADPβS > dATP > UTP. In the presence of Mg 2+ , the K ATP ‐channels were activated by dADP, GTP, GDP and UDP. 4 Tolbutamide inhibited the K ATP ‐channels not only in the presence but also in the prolonged absence of Mg 2+ . In nucleotide‐free solutions, the potency of tolbutamide was very low. When about half of the K ATP ‐channel activity was inhibited by ATP, AMP‐PNP, ADPβS or ADP (absence of Mg 2+ ), the potency of tolbutamide was increased. 5 Tolbutamide (100 μ m ) slightly enhanced the channel‐inhibiting potency of AMP‐PNP and inhibited the channel‐activating effect of MgGDP in a non‐competitive manner. 6 Channel activation by MgGDP (0.5 mM) competitively antagonized the inhibitory responses to AMP‐PNP (1 μ m ‐1 mM). This effect of GDP was neutralized by tolbutamide (100 μ m ). 7 The stimulatory effect of 0.5 mM MgGDP was neutralized by 200 μ m AMP‐PNP. Under these conditions the potency of tolbutamide was much higher than in the presence of 0.5 mM MgGDP alone or in the absence of any nucleotides. 8 dADP (0.3–1 mM) increased the potency of tolbutamide. Additional application of 200 μ m AMP‐PNP caused a further increase in the potency of tolbutamide. 9 In conclusion, in the simultaneous presence of inhibitory and stimulatory nucleotides, binding of sulphonylureas to their receptor causes direct inhibition of channel activity, non‐competitive inhibition of the action of stimulatory nucleotides and interruption of the competitive interaction between stimulatory and inhibitory nucleotides. The latter effect increases the proportion of K ATP ‐channels staying in the nucleotide‐blocked state. In addition, this state potentiates the direct effect of sulphonylureas.