Distinguishing surface effects of calcium ion from pore-occupancy effects in Na+ channels

The effects of calcium ion on the Na+ activation gate were studied in squid giant axons. Saxitoxin (STX) was used to block ion entry into Na+ channels without hindering access to the membrane surface, making it possible to distinguish surface effects of calcium from pore-occupancy effects. In the presence of STX, gating kinetics were measured from gating current (Ig). The kinetic effects of external calcium concentration changes were small when STX was present. In the absence of STX, lowering the calcium concentration (from 100 to 10 mM) slowed the closing of Na+ channels (measured from INa tails) by more than a factor of 2. Surprisingly, the voltage sensitivity of closing kinetics changed with calcium concentration, and it was modified by STX. Voltage sensitivity apparently depends in part on the ability of calcium to enter and block the channels as voltage is driven negative. In external medium with no added calcium, INa tail current initially increases in amplitude severalfold with the relief of calcium block, then progressively slows and gets smaller, as calcium diffuses out of the layers investing the axon. INa tails seen just before the current disappears suggest that closing in the absence of channel block is very slow or does not occur. INa amplitude and kinetics are completely restored when calcium is returned. The results strongly suggest that calcium occupancy is a requirement for channel closing and that nonoccupied channels fold reversibly into a nonfunctional conformation.

Correlates of the desired family size among Indian communities.

The People of India database of the Anthropological Survey of India documents 631 cultural, ecological, and economic traits of the 4635 communities to which the entire Indian population is assigned. Focusing on 1342 communities of South India, we looked for correlates of low (1 or 2 children) and high (4 or more children) desired family size (DFS) reported as the norm for any given community by key informants. We found 10 cultural and 18 economic traits to be significantly correlated to high DFS and 21 cultural and 9 economic traits to low DFS. The economic traits so identified are compatible with high family size being desired by parents who have little capability of investing in quality of offspring, but whose children contribute economically from an early age. In contrast, communities desiring low family size are part of the modern intensive agriculture/organized industry/services sector and invest heavily in educating their children. A composite index based on 27 economic traits (CEI) has a high predictive value with respect to the DFS for the entire set of 4635 Indian communities. The 31 cultural traits highly correlated to high or low DFS constitute 5 clusters that can be identified as characterizing scheduled tribes, scheduled castes, rural and landless lower castes, urban upper castes, and Moslems. Whereas economic traits have similar influence on DFS within each of these ethnic categories, Moslems demonstrate a significantly higher DFS for lower values of CEI.