Anion dependence of the contractions of skinned cardiac cells

Adenosine triphosphate and the late steps in calcium-dependent exocytosis at a ribbon synapse

The ATP dependence of the kinetics of Ca2+-dependent exocytosis after flash photolysis of caged Ca2+ was studied by capacitance measurements with submillisecond resolution in single synaptic terminals of retinal bipolar neurons. After control experiments verified that this combination of techniques is valid for the study of exocytosis in synaptic terminals, a comparison was made between the Ca2+ dependence of the rate of exocytosis in synaptic terminals internally dialyzed with MgATP, MgATP-gamma-S, or no added Mg2+ or nucleotide. The Ca2+ threshold for release, the maximum rate of release, and the overall relationship between the rate of synaptic vesicle fusion and [Ca2+]i were found to be independent of MgATP. A decrease in the average rate at near-threshold [Ca2+]i was observed in terminals with MgATP-gamma-S, but due to the small sample size is of unclear significance. The Ca2+ dependence of the delay between the elevation of [Ca2+]i and the beginning of the capacitance rise was also found to be independent of MgATP. In contrast, MgATP had a marked effect on the ability of terminals to respond to multiple stimuli. Terminals with MgATP typically exhibited a capacitance increase to a second stimulus that was >70% of the amplitude of the first response and to a third stimulus with a response amplitude that was >50% of the first, whereas terminals without MgATP responded to a second stimulus with a response <35% of the first and rarely responded to a third flash. These results suggest a major role for MgATP in preparing synaptic vesicles for fusion, but indicate that cytosolic MgATP may have little role in events downstream of calcium entry, provided that [Ca2+]i near release sites is elevated above approximately 30 microM.

Dual effects of calcium on ATP-sensitive potassium channels of frog skeletal muscle

The block of ATP-sensitive K+ channels by ATP were studied in inside-out patches of frog toe muscles. Seal formation within Ca(2+)-free Ringer induces many open channels which can be irreversibly blocked by subsequent addition of Ca2+. However, such Ca(2+)-blocked and ATP-blocked channels can be influenced by the addition of ATP and Ca2+: the block by ATP is partially but irreversibly cancelled.

Effects of potassium channel openers on single potassium channels in mouse skeletal muscle

The patch-clamp technique was used to study the effects of the potassium channel openers cromakalim, pinacidil, RP 49356 and diazoxide on single potassium channels in mouse skeletal muscle. In excised patches in the inside-out configuration, one type of potassium channel, the ATP-sensitive potassium channel, could be activated by internally applied RP 49356 even in the absence of internal ATP. At a concentration of 0.4 and 0.8 mmol/l, RP 49356 increased the open-probability of the channels by a factor of 2.7 and 17.4 respectively. The stimulating effect of cromakalim (0.2-0.8 mmol/l) and pinacidil (0.4 mmol/l) depended on the presence of ATP (0.1 mmol/l) at the cytoplasmic side of the patch membrane. The two drugs were able to restore the open-probability of the channels blocked by internal ATP (0.1 mmol/l) to 50-90% of its value in ATP-free solution. No channel reactivation could be observed at a higher ATP concentration (1 mmol/l). Diazoxide (0.4 mmol/l) had almost no effect. None of these channel openers could stimulate the other prominent type of potassium channel in skeletal muscle, the large-conductance Ca2(+)-activated potassium channel. The results show that cromakalim, pinacidil and RP 49356 are specific openers of ATP-sensitive potassium channels in skeletal muscle. It is suggested that the drugs displace the channel blocker ATP and that RP 49356 in addition recruits inactive channels.