Calcium transport in the lens.

Area of Science:

• Ocular Physiology
• Cellular Transport Mechanisms
• Biochemistry

Background:

• Mammalian lens calcium concentration is significantly lower than in the aqueous humor.
• This concentration gradient suggests an active calcium transport system is present.

Purpose of the Study:

• To investigate the mechanism responsible for maintaining low calcium levels in the mammalian lens.
• To identify and characterize the calcium transport system.

Main Methods:

• Measurement of calcium (45Ca) efflux rates at different temperatures (4°C vs. 37°C).
• Assessment of calcium efflux in the absence of external sodium (Na+).
• Identification and characterization of calcium-activated, magnesium-dependent ATPase activity in lens homogenates.
• Testing the effects of inhibitors like lanthanum and propranolol on calcium efflux and ATPase activity.

Main Results:

• Lens calcium concentration was found to be approximately 0.2 mM, an order of magnitude lower than aqueous humor.
• Calcium efflux was highly temperature-dependent, decreasing by 85% at 4°C compared to 37°C.
• Calcium efflux was not affected by the absence of external Na+.
• Inhibitors of Ca/Mg ATPase, lanthanum and propranolol, significantly reduced calcium efflux.
• Ca/Mg ATPase activity was confirmed in rat, bovine, and rabbit lenses and was inhibited by lanthanum and propranolol.

Conclusions:

• Evidence strongly supports the existence of an active calcium transport system in the mammalian lens.
• A calcium-activated, magnesium-dependent ATPase is likely the key component of this system.
• This ATPase plays a critical role in regulating lens calcium homeostasis.