[Ca2+ homeostasis in mammalian cells]

Area of Science:

• Cellular Physiology
• Muscle Contraction
• Calcium Signaling

Background:

• Cytoplasmic free calcium concentration ([Ca2+]i) is a critical regulator of cellular activity, particularly in muscle.
• Understanding the dynamics of [Ca2+]i during excitation-contraction coupling is essential for comprehending muscle function.

Purpose of the Study:

• To investigate changes in [Ca2+]i during muscle excitation using optical techniques.
• To explore the relationship between [Ca2+]i, myosin light chain phosphorylation, and force development in smooth muscle.
• To elucidate novel regulatory mechanisms of calcium handling and signaling within cells.

Main Methods:

• Optical techniques for monitoring [Ca2+]i in intact smooth muscle preparations.
• Investigation of excitation-contraction coupling, including myosin light chain phosphorylation and force development.
• Confocal laser microscopy to study intracellular calcium gradients.

Main Results:

• Observed that [Ca2+]i and force development are not always tightly coupled during muscle stimulation.
• Demonstrated that agonists can modulate the sensitivity of the contractile apparatus to calcium.
• Revealed that Ins(1,4,5)P3 receptor activity is controlled by luminal Ca2+ and cellular redox status.
• Showed stimulation of the plasma-membrane Ca2+ pump by negatively charged phospholipids.
• Identified spatio-temporal differences in Ca2+ distribution between nucleus and cytosol, suggesting nuclear calcium regulation.

Conclusions:

• Calcium's role in smooth muscle contraction is complex, with factors beyond concentration influencing force.
• Internal calcium stores and their release channels are subject to intricate regulation by luminal calcium and redox state.
• The plasma membrane calcium pump's activity is modulated by phospholipids.
• Distinct calcium gradients and regulatory mechanisms exist at the nuclear envelope, impacting nuclear calcium signaling.