Intracellular pH (pHi) regulation was studied in acid-loaded barnacle muscle fibers by monitoring recovery of pHi with a pH-sensitive microelectrode. By multiplying the rate of pHi recovery by total intracellular buffering power, the acid extrusion rate was obtained. The acid extrusion rate was greatest at low values of pHi, and declined toward zero as pHi approached normal levels. It increased as the extracellular pH (pHo) was raised either by increasing external [HCO3] ([HCO3]o) at constant PCO2 or by decreasing PCO2 at constant [HCO3]o, but more so in the former case than in the latter. These observations suggest that pHo per se is an important determinant of the acid extrusion rate, but that raising [HCO3]o by itself also stimulates acid extrusion. This would be expected if acid extrusion involves the inward movement of HCO3. When fibers were exposed to HCO3-containing solutions at very low or very high pHo, pHi drifted downward or upward, respectively; thbe drifts were inhibited by 4-acetamido-4' isothiocyanostilbene-2,2' disulfonic acid (SITS). Our results are discussed in terms of possible mechanisms of acid extrusion.

Intracellular pH (pHi) regulation was studied in barnacle muscle fibers with pH-sensitive microelectrodes. The cells were acid loaded, and the subsequent recovery of pHi was monitored. The rate of recovery was reduced by one-third when external Na+ ([Na+]o) was replaced by Li+, but recovery was completely abolished when Na+ was replaced by choline or N-methyl-D-glucamine. In other experiments, varying amounts of Na+ were replaced by choline, and the acid extrusion rate, derived from the recovery rate of pHi, was calculated at a single value of pHi, 6.80. The dependence of the acid extrusion rate on [Na+]o could be described by Michaelis-Menten kinetics; at pHo (extracellular) = 8.0 and [HCO3-]o (extracellular) = 10 mM, the apparent Km and Vmax were 59 mM and 1.3 mmol x l(-1) x min-1. When [HCO3-]o was reduced to 2.5 mM at the same pHo, Km did not change significantly, but Vmax was substantially reduced. On the other hand, when pHo was reduced to 7.4 at constant [HCO3-]o, Vmax changed only slightly, but Km increased substantially. In similar experiments, we examined the dependence of the acid extrusion rate on [HCO3-]o. At pHo = 8.0 and [Na+]o = 440 mM, the apparent Km and Vmax were 4.1 mM and 2.1 mmol x 1-1 x min-1. When pHo was reduced to 7.4, Vmax was not altered, but Km substantially increased. The kinetic data are discussed in terms of the role of pHo, [Na+]o, and [HCO3-]o in the pHi-regulating system.

Native forages of the southern United States are frequently low in nutrients and poorly digestible, while improved pastures are the opposite. Since this area produces rapid growth of pines, which is a major use of the land, the integration of pines, pastures, and cattle seem to offer an efficient and economical form of management. Therefore, in a 20-year study Coastal bermuda (Cynodon dactylon), dallis (Paspalum dilatatum), and Pensacola bahia (P. notatum) grasses were grown in pastures with no trees, and with slash pine (Pinus elliottii) spaced 3.7 × 3.7 and 6.1 × 6.1 m. The trees were grown free of competition for 3 years, grasses were established during the 4th year, and grazing by yearling cattle began the 5th year. The pastures were then fertilized, grazed, and burned for 15 years. Increasing tree canopies reduced forage production of all grasses. Bahiagrass was the most shade tolerant, while Coastal bermuda was the least. Liveweight gains during the 15 years of grazing totaled 3933, 2302, and 1518kg/ha from pastures with no pines and slash pine spaced 6.1 × 6.1 and 3.7 × 3.7, respectively. Survival of slash pine was low due to heavy attacks by southern pine coneworm and southern fusfform rust. However, the trees grew rapidly and averaged 19.0 and 16.9 m in height, 30.0 and 33.3 cm in diameter, and 181.4 and 90.7 m3/ha of pulpwood after 20 years from the 3.7 × 3.7 and 6.1 X 6.1 spacings, respectively. Slash pine at these spacings planted in undisturbed, native vegetation were 16.0 and 15.7 m tall, 19.6 and 23.4 cm in diameter, and produced 145.5 and 63.6 m3/ha of pulpwood. Combining the production of pines and beef on improved pasture offers an opportunity for multipleproduct yields. Since this approach to agroforestry is not trouble-free, good management is required. Landowners, especially those with small holdings, should consider these alternatives to single-product management when planning their cropping system.

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