Related Experiment Videos
Calcium influx in contracting and paralyzed frog twitch muscle fibers
Abstract:
Calcium uptake produced by a potassium contracture in isolated frog twitch fibers was 6.7 +/- 0.8 pmol in 0.7 cm of fiber (mean +/- SEM, 21 observations) in the presence of 30 microM D600. When potassium was applied to fibers paralyzed by the combination of 30 microM D600, cold, and a prior contracture, the calcium uptake fell to 3.0 +/- 0.7 pmol (11): the fibers were soaked in 45Ca in sodium Ringer for 3 min before 45Ca, in a potassium solution, was added for 2 min; each estimate of uptake was corrected for 5 min of resting influx, measured from the same fiber (average = 2.3 +/- 0.3 pmol). The calcium influx into paralyzed fibers is unrelated to contraction. This voltage-sensitive, slowly inactivating influx, which can be blocked by 4 mM nickel, has properties similar to the calcium current described by several laboratories. The paired difference in calcium uptake between contracting and paralyzed fibers, 2.9 +/- 0.8 pmol (16), is a component of influx related to contraction. Its size varies with contracture size and it occurs after tension production: 45Ca applied immediately after contracture is taken up in essentially the same amounts as 45Ca added before contraction. This delayed uptake is probably a "reflux" refilling a binding site on the cytoplasmic side of the T membrane, which had been emptied during the prior contracture, perhaps to initiate it. We detect no component of calcium uptake related to excitation-contraction coupling occurring before or during a contracture.
Insights
Calcium uptake in frog muscle fibers is primarily a delayed "reflux" after contraction, not directly linked to excitation-contraction coupling during the event itself. This finding clarifies calcium
Area of Science:
- Muscle Physiology
- Cellular Biology
- Calcium Signaling
Background:
- Potassium contractures in isolated frog twitch fibers are known to involve calcium uptake.
- The precise timing and mechanism of calcium influx related to excitation-contraction coupling remain under investigation.
Purpose of the Study:
- To investigate the temporal relationship between calcium uptake and muscle contraction in isolated frog twitch fibers.
- To differentiate calcium influx components associated with contraction from those occurring independently.
Main Methods:
- Isolated frog twitch fibers were exposed to potassium-induced contractures.
- Radioactive calcium-45 (45Ca) uptake was measured in contracting and paralyzed fibers under varying conditions.
- Uptake measurements were corrected for resting influx and compared between experimental groups.
Main Results:
- Calcium uptake during potassium contracture was significantly reduced in paralyzed fibers (3.0 pmol) compared to contracting fibers (6.7 pmol).
- A component of calcium uptake related to contraction (2.9 pmol) was identified, occurring after tension production.
- This delayed uptake exhibits properties suggesting a "reflux" mechanism refilling T-tubule binding sites emptied during contraction.
Conclusions:
- Calcium uptake related to excitation-contraction coupling does not appear to occur before or during a contracture in these fibers.
- The observed calcium influx is voltage-sensitive, slowly inactivating, and blocked by nickel, resembling known calcium currents.
- A significant portion of calcium uptake following contraction is likely a delayed refilling process, not directly coupled to the initiation of the contraction itself.