The action of thapsigargin on calcium channels in smooth muscle cells

Since 1883 the role of calcium on the smooth muscle has been know. Its main function is to trigger and sustain the contraction.
In a smooth muscle cell intracellular free Ca2+ concentration is kept very low by a low permeability of the plasma membrane and by different Ca2+ transporters. One of these transporters is the Ca2+-ATPase of the sarcoplasmic reticulum. Its role is to recapture the Ca2+ liberated from the stores after a contraction. Another important element for a smooth muscle cell is the voltage-operated Ca2+ channel which allows the rapid entry of Ca2+ needed for a contraction.
Thapsogargin, a sesquiterpene lactone, is often use as a pharmacological tool for studying the intracellular Ca2+ storage and release processes. It inhibits specifically the Ca2+-ATPas of the sarcoplasmic reticulum at nanomolar concentrations. At micromolar concentrations several authors noted a decrease in the current through voltage-operated Ca2+ channels. Ganitkevich and Isenberg (1992) proposed that decrease in the current, in myocytes from guinea-pig urinary bladder, was an indirect effect of thapsigargin, that the rise in [Ca2+]I due to the block of the sarcoplasmic reticulum Ca2+ pump causing a Ca2+-induced inactivation of the Ca2+ channel. Rosier et al (1993) also observed this same decrease in the Ca2+ channel current in adrenal glomerulosa cells but concluded that thapsigargin had a direct effect in the Ca2+ channels. They stressed that the action of thapsigargin at micromolar concentrations should be interpreted with caution.
The purpose of this thesis was to determine the exact effect of thapsigargin at micromolar concentrations on the voltage-operated Ca2+ channels in vascular smooth muscle cells.
This phenomenon was verified using the electrophysiological method of the whole-cell patch-clamp technique enabling the direct study of an antagonist on the voltage-operated Ca2+ channel in a cell line, A7r5 cells derived from the thoracic aorta of embryonic DBAX rats.
Thapsigargin has a direct and irreversible effect on the voltage-operated Ca2+ channels at micromolar concentrations. It decreases the current through these channels in a voltage-independent manner. An interesting phenomenon was seen, thapsigargin seems to act differently depending ont the channel being slow or fast inactivating. The reason for this is not clear and would need further investigation