The bone compartment in a teleost fish, Ictalurus punctatus: size, composition and acid-base response to hypercapnia

An estimate of the total mass of bone in the Channel catfish Ictalurus punctatus Rafinesque, was obtained by dissection. The wet weight of bone constituted 16.3 +/- 1.9% (+/- S.D.) of the total (live) wet weight, and 25.0 +/- 2.1% of the dry weight. Of the dry skeletal material, 66.3 +/- 11.1% was soluble in strong acid. The acid-soluble material was about half mineral salts, consisting of 19.5 +/- 2.21% Ca2+ and 27.6 +/- 3.22% PO4(3-), with minor fractions of Mg2+ (0.33%) and CO3(2-) (1.67%). The pH values of fluid compartments associated with skull and vertebral bone tissues were 7.420 +/- 0.026 and 7.444 +/- 0.017 (+/- S.E.), respectively, at a normocapnic plasma pH of 7.868 +/- 0.020. In response to external hypercapnia (7.5 Torr), the blood response consisted of an immediate decrease in pH, and a subsequent compensatory rise in both pH and [HCO3-]. This compensatory phase was accompanied by a net apparent H+ excretion to the water. The participation of the mineral salts of the bone compartment in compensation appeared to be negligible, since there was no significant change in either blood [Ca2+] or [PO4(3-)], nor any significant increase in calcium efflux to the water. The intracellular pH values of the bone compartments were only slightly higher than other tissues, and the changes in pHi during hypercapnia were similar in bone and white muscle. Thus, the bone compartment in the fish appears to be well regulated, relatively refractory to acute acid-base disturbance, and does not serve as an ion source during acid-base compensation.