ONTOGENY OF THE RYANODINE RECEPTOR IN RABBIT URINARY-BLADDER SMOOTH-MUSCLE

Bladder smooth muscle contraction is mediated by both direct calcium e ntry through the cell membrane, and by calcium induced calcium release (CICR) from the sarcoplasmic reticulum (SR) storage sites. Ryanodine is a neutral plant alkaloid which binds to an ion channel located on t he SR membrane. Its effects in cardiac skeletal muscle are well charac terized where it inibits the efflux of intracellular calcium stores, a nd thus it serves as a negative inotrope. It has also been shown that in the develpping rabbit myocardium, there is a gradual increase in th e expression of this ion channel. Little has been written about the ex pression and function of the ryanodine sensitive ion channel in smooth muscle. Recently we have shown that neonatal rabbit bladder smooth mu scle is not very sensitive to ryanodine, while that from mature rabbit s is extremely sensitive. This leads us to quantify the expression oft he ryanodine sensitive ion channel. In this paper we demonstrate that the Kd values do not change to any significant degree with normal rabb it bladder development. However the Bmax values for 3 day, 2, 4, 6, an d 8 week rabbit bladder smooth muscle are 7, 10, 15, 29, and 44 fmol s pecifically bound ryanodine/mg protein. The differences between the ne onatal groups and the mature groups are significant (P<0.5). This incr ease in ryanodine sensitive ion channel expression with normal growth would suggest that with normal maturation, the bladder smooth muscle c ell acquires an increased pool of sequestrered intracellular calcium. This would follow a similar pattern of development that has already be en described in rabbit myocardium.