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Quantitative studies on plasmalemmal folds and caveolae of rabbit ventricular myocardial cells
Abstract:
Plasmalemmal folds and caveolae were investigated by qualitative and quantitative analysis of electron micrographs obtained by freeze fracture and transmission electron microscopy (TEM) of rabbit right ventricular papillary muscles whose mean sarcomere lengths ranged from 1.64 to 2.28 micron. In passively extended muscles, folds were observed at sarcomere lengths of 2.3 micron and could be shown by extrapolation to become completely extended at a maximum sarcomere length of 2.8 micron. It was concluded that the plasmalemma does not contribute to resting tension in the physiological range of sarcomere lengths. Caveolae are present in both the external plasmalemmal envelope and T-tubular plasmalemma. They show no preferential distribution with respect to underlying myofibrillar striations or membrane folds and are nearly devoid of membrane particles in freeze-fractured material. The surface density of caveolar necks (4.0/micron2 apparent plasmalemmal area) is only 16-20% of that reported for frog skeletal muscle. Caveolae augment plasmalemmal area by 21-32%, assuming two or three caveolae per neck, respectively. Caveolar membrane does not serve as a reservoir of membrane to be recruited into external plasmalemma, at least over the physiological range of sarcomere lengths. In heart muscle they do not account for the T-tubular access resistance, and their function in this tissue remains unknown.
Insights
Rabbit heart muscle plasmalemma folds do not contribute to resting tension. Caveolae in cardiac muscle do not serve as a membrane reservoir and their function remains unknown.
Area of Science:
- Cardiovascular Biology
- Cell Biology
- Muscle Physiology
Background:
- Plasmalemmal folds and caveolae are key membrane structures in muscle cells.
- Understanding their role in cardiac muscle is crucial for comprehending muscle mechanics and function.
Purpose of the Study:
- To investigate the structural characteristics and functional implications of plasmalemmal folds and caveolae in rabbit ventricular papillary muscles.
- To determine the contribution of plasmalemmal folds to resting tension and the role of caveolae in membrane dynamics.
Main Methods:
- Qualitative and quantitative analysis of electron micrographs.
- Utilized freeze fracture and transmission electron microscopy (TEM).
- Examined rabbit right ventricular papillary muscles at various sarcomere lengths.
Main Results:
- Plasmalemmal folds fully extend at sarcomere lengths beyond 2.8 microns, indicating no contribution to resting tension within the physiological range.
- Caveolae are present in both external and T-tubular membranes, with no preferential distribution.
- Caveolar surface density is lower than in skeletal muscle, and they do not appear to function as a membrane reservoir.
Conclusions:
- The plasmalemma does not contribute to resting tension in the physiological sarcomere length range of cardiac muscle.
- Caveolae in cardiac muscle do not account for T-tubular access resistance, and their specific function in this tissue remains undetermined.