CONVERSION BETWEEN PERMEABILITY STATES OF IP3 RECEPTORS IN CULTURED SMOOTH-MUSCLE CELLS

The kinetics of the effect of inositol 1,4,5-trisphosphate (IP3) on Ca 2+ in the sarcoplasmic reticulum (SR) were studied in saponin-permeabi lized A7r5 cells. At 0.1 mu M, IP3 elicited slow monoexponential decli nes in SR free Ca2+ concentration ([Ca2+]SR) For IP3 concentration ([I P3]) = 0.2-100 mu M, evoked declines in [Ca2+]SR were biphasic and bes t fit as the sum of two first-order processes with rate constants k(fa st) and k(slow). The k(fast) varied as a function of [IP3] over the ra nge tested, whereas k(slow) was already maximal when [IP3] = 0.1 mu M. TO analyze SR Ca2+ release elicited by 1P(3), the rate constants for IP3-induced changes in the total SR Ca2+ content (k(R)) were calculate d. k(R) was accurately described only when both [Ca2+](SR) and [IP3] w ere considered together. k(R) was dependent on IP3 binding to receptor s that existed in either of two states, a high-affinity low-conductanc e state (IP(3)R(H)) and a low-affinity high-conductance state (IP(3)R( L)) The permeability of IP(3)R(L) was 12.28 times larger than that of IP(3)R(H), and the conversion between permeability states as well as c hanges in both the affinity and cooperativity with which IP3 was bound to IP(3)R(L) were mediated by SR Ca2+. This SR Ca2+-dependent modulat ion of the characteristics of IP3 receptors forms the basis for the bi phasic time course characteristic of IP3-evoked SR Ca2+ release.