Ca2+ signalling and Ca2+-activated K+ channels in smooth muscle

Area of Science:

• Physiology
• Cellular Electrophysiology

Background:

• Smooth muscle cells generate spontaneous transient outward currents (STOCs) mediated by calcium-activated potassium (KCa) channels.
• STOCs are triggered by transient subsarcolemmal increases in intracellular calcium (Ca2+) originating from the sarcoplasmic reticulum.

Purpose of the Study:

• To investigate if STOCs are spatially restricted membrane currents.
• To determine if single KCa channel activity is regulated by bulk cytosolic Ca2+ concentration changes.
• To examine the relationship between voltage-dependent calcium current (ICa) and KCa channel activity.

Main Methods:

• Whole-cell voltage clamp of guinea-pig colonic myocytes to measure macroscopic currents.
• Simultaneous measurement of bulk average cytosolic Ca2+ concentration ([Ca2+]c) using Fura-2.
• Cell-attached patch recordings to assess single KCa channel activity.

Main Results:

• Single KCa channel voltage-dependence differed from STOC activity, supporting the spatial restriction of STOCs.
• Depolarization increased [Ca2+]c, STOCs, and single KCa channel activity, with distinct voltage sensitivities.
• KCa channel activity, but not bulk [Ca2+]c, increased when only the patch was depolarized, indicating local Ca2+ sensitivity.
• The time course of KCa channel activity mirrored ICa, suggesting Ca2+ influx as a trigger.

Conclusions:

• STOCs are spatially restricted membrane currents, not global events.
• KCa channels are activated by local subsarcolemmal Ca2+ increases and membrane depolarization.
• KCa channel activity is primarily influenced by local Ca2+ transients, not bulk cytosolic Ca2+ levels.