Variable luminal sarcoplasmic reticulum Ca(2+) buffer capacity in smooth muscle cells.
Sarcoplasmic reticulum contains the internal Ca(2+) store in smooth muscle cells and its lumen appears to be a continuum that lacks diffusion barriers. Accordingly, the free luminal Ca(2+) level is the same all throughout the SR; however, whether the Ca(2+) buffer capacity is the same in all the SR is unknown. We have estimated indirectly the luminal Ca(2+) buffer capacity of the SR by comparing the reduction in SR Ca(2+) levels with the corresponding increase in [Ca(2+)](i) during activation of either IP(3)Rs with carbachol or RyRs with caffeine, in smooth muscle cells from guinea pig urinary bladder. We have determined that carbachol-sensitive SR has a 2.4 times larger Ca(2+) buffer capacity than caffeine-sensitive SR. Rapid inhibition of SERCA pumps with thapsigargin revealed that this pump activity accounts for 80% and 60% of the Ca(2+) buffer capacities of carbachol- and caffeine-sensitive SR, respectively. Moreover, the Ca(2+) buffer capacity of carbachol-sensitive SR was similar to caffeine-sensitive SR when SERCA pumps were inhibited. Similar rates of Ca(2+) replenishments suggest similar levels of SERCA pump activities for either carbachol- or caffeine-sensitive SR. Paired pulses of caffeine, in conditions of low Ca(2+) influx, indicate the relevance of luminal SR Ca(2+) buffer capacity in the [Ca(2+)](i) response. To further study the importance of luminal SR Ca(2+) buffer capacity in the release process we used low levels of heparin to partially inhibit IP(3)Rs. This condition revealed carbachol-induced transient increase of luminal SR Ca(2+) levels provided that SERCA pumps were active. It thus appears that SERCA pump activity keeps the luminal SR Ca(2+)-binding proteins in the high-capacity, low-affinity conformation, particularly for IP(3)R-mediated Ca(2+) release.
Dagnino-Acosta
Adán
A
Department of Physiology, Biophysics and Neurosciences, CINVESTAV, D.F., Mexico.
Guerrero-Hernández
Agustín
A
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D002217:Carbachol / Q000494:pharmacology; D006168:Guinea Pigs; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000378:metabolism; D032389:Myocytes, Smooth Muscle / Q000378:metabolism*; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000378:metabolism; D019284:Thapsigargin / Q000494:pharmacology
D053496:Inositol 1,4,5-Trisphosphate Receptors; D019837:Ryanodine Receptor Calcium Release Channel; D002110:Caffeine; D019284:Thapsigargin; D002217:Carbachol; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D002118:Calcium