There is increasing evidence that
gamma-sarcoglycan is absent and other
sarcoglycans are reduced in patients with the
limb-girdle muscular dystrophy type 2C (
LGMD2C) form of severe childhood autosomal recessive
muscular dystrophy. In the present investigation, we combined microspectrofluorimetry and electron microscopy techniques to investigate the physiological function and the ultrastructure of control and
LGMD2C myotubes. Results obtained from Ca2+ measurements showed that the resting level of the cytosolic free
calcium ([Ca2+ ]i ) in control myotubes was 73+/-3.4 nmol/l (mean+/-se, n=35) and in
LGMD2C myotubes was 69+/-4 nmol/l (n=44).
Carbachol (CCh, 10 micromol/l ) induced a 335+/-10 nmol/l (n=8) rise in [Ca2+ ]i in control myotubes and 531.9+/-32 nmol/l (n=23) in
LGMD2C myotubes. Similarly, elevations of [Ca2+ ]i by 35 mmol/l K+ were 324+/-32 nmol/l (n=8) in control myotubes and 442.8+/-24 nmol/l (n=22) in
LGMD2C myotubes.
Caffeine (10 mmol/l) activated similar [Ca2+]i peaks in control and
LGMD2C myotubes but induced a biphasic response in
LGMD2C in four out of 12 myotubes and only a monophasic response in control myotubes. The ultrastructural results showed that the plasma membrane was abnormally indented and convoluted in both the
LGMD2C biopsy and the
LGMD2C cultured myotubes. It is suggested that the reduction in components of the
dystrophin-
glycoprotein complex results in the instability and an increase in the surface area of the plasma membrane, which may result in a higher population of Ca2+ channels in the
LGMD2C myotubes.