Correlation between apical intramembrane particles and H+ secretion rates during CO2 stimulation in turtle bladder

To correlate the prevalence of rod-shaped intramembrane particles (RSP) in the apical membranes of carbonic anhydrase-rich (CA) cells and the H+ transport rate in turtle urinary bladder, we carried out morphometric studies by means of scanning and freeze-fracture electron microscopy of the alpha and beta subpopulations of CA cells. Correlations were made between the apical membrane areas of alpha cells and H+ transport rate at 0 and 5% ambient CO2. Exposure to CO2 more than doubled the planar area of the luminal surface of alpha cells and increased the degree of folding (amplification) of the apical cell membrane from 2.8 +/- 0.3 to 3.8 +/- 0.3. The actual apical membrane area of alpha cells increased from 176 mm2 to 693 mm2 per 8 cm2 epithelial area. The RSP density also appeared to be increased by about 40%. The total CO2-induced increase in RSPs in position at the luminal surface was 5 fold while the increase in H+ transport was 9-fold. We conclude that stimulation of H+ transport by CO2 involves recruitment of RSP to the apical cell membrane of alpha-type CA cells and that RSPs are associated with active H+ transport. They may represent linear arrays of transmembrane components of H+ pumps.