In Alkalosis, Hydrogen ions are depleted. Those ions are normally bound to plasma proteins like albumin and usually compete with Calcium ions for these proteins. Since Hydrogen ions are depleted, Calcium levels bound by the proteins will be increased thereby decreasing free (unbound) calcium level resulting in tetany.
In case of alkalosis..the blood pH increases..the protein in the blood becomes more ionized into anions.This causes the free calcium present in blood to bind strongly with protein. If severe, it may cause tetany (alkalotic tetany).
becouse of less hvdrogen ions on the COOH groups of acidic aminoacids, Charge of COOH groups is converted to COO- :this extra negative charges bind the ionized calcium
Total serum calcium remains normal but ionized calcium is reduced this lead to increase in PTH, PTH tend to increase ionised calcium level but it can not prevent binding of ionised calcium to negatively charged amino acids.....so..we r left with Hypocalcemia