Use of 3H-QNB in the isolation of plasma membrane from smooth muscle of the urinary bladder: effect of oxalate on calcium uptake by the membrane fractions

Specific binding of tritiated quinuclidinyl benzilate (3H-QNB) to surface membrane muscarinic receptors was utilized to identify plasma membrane (PM) fractions from smooth muscle of the rabbit urinary bladder. Accumulation of 3H-QNB in the PM fraction was 4-5-fold higher than that in fractions of endoplasmic reticulum (EM) or mitochondria (M). A similar pattern of distribution was found for 5'-nucleotidase. 3H-QNB binding therefore appears to be a suitable marker for plasma membrane of the urinary bladder. Data on ATP-dependent calcium uptake by PM and ER fractions showed that oxalate highly potentiated calcium uptake by both fractions and consequently this feature cannot be used to identify ER fractions specifically.