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In the rat left atrium, electrically driven, a superfusion dropping method was developed. The device essentially consisted of a pair of platinum wire electrodes which held one end of the tissue, while the other end was attached to an isometric tension transducer. Either normal or isotopically labeled solutions were poured on the preparation at preset flow rates by means of a minipump. Effluent was collected fractionally in vials for measurement of radioactivity. Several perfusion flows tested led us to the conclusion that higher rates than 2.26 ml/min did not improve contractile performance. Stability of the preparation was found similar to the classical isolated organ-bath method during a 180 min period of observation. Set-up of the superfused atria reproduced paired determinations of 45Ca efflux, together with the contractile parameters. In addition, the frequency of sampling (intervals of 1 min or less) allowed us to detect changes in the washout pattern of exchangeable 45Ca even during early phases of Ca efflux. Therefore, the method reported herein seems to overcome the serious handicap of the scanty tissue employed.

The effects of arginine vasopressin (VP), 8 mU/ml, on contractility, heat production and glucose metabolism were investigated in isolated, arterially perfused, interventricular rabbit septa. Rest tension linearly increased in the presence of VP with a slope of 0.095 +/- 0.008 g/min. Developed tension (DT), maximal rate of tension development (+dT/dt)max and maximal rate of relaxation (-dT/dt)max showed a biphasic response in the presence of VP. The first phase showed a rapid decline in all three contractile parameters [DT, (+dT/dt)max and (-dT/dt)max] with a half time of 4.4, 3.3 and 4.6 min, respectively. This rapid decline was followed by a recovery period that occurs with an increase in glycolytic flux and on lactic acid production. Along with the effects on contractile parameters, VP increases the ratio heat production over developed tension (40% over control values), indicating that in the presence of VP "muscle efficiency" decreases since it is energetically more costly to generate a given level of isometric tension. This was further confirmed by experiments performed under low Ca conditions (150 microM Ca Cl2) in which, while VP showed no significant changes on contractile parameters, myocardial heat production and heat production over developed tension increased.

The sodium (Na) fractions of the dog carotid artery were identified by analyzing the effects of different procedures on 24Na washout curves. Although these never yielded more than three exponential terms, four Na fractions were identified. The fast-exchanging component amounted to 73 mmoles Na/kg w.wt., followed the kinetics of diffusion in a flat sheet with a diffusion coefficient of 6.9 X 10(-6) cm2 X s-1, and remained unaltered under the different experimental conditions. The intermediate component exchanged with a half time (t0.5) of 5.3 min and amounted to 18 mmoles Na/kg w.wt. About one third of this (6 mmoles) was constituted by cellular Na and the remaining by non-cellular Na, probably associated with extracellular structures. These two fractions could be discriminated by: (a) decreasing the rate of exchange of the cellular fraction with ouabain or low temperature (17 degrees C); (b) cell damage by freezing and thawing combined with metabolic poisoning; (c) enzymatic digestion of extracellular structures. When procedures (a) and (c) were combined, the intermediate component entirely disappeared. A third residual component amounted to 0.6 mmoles Na/kg w.wt and exchanged with a t0.5 of 70 min. It was unaffected by cellular damage or enzymatic digestion and was masked by the cellular phase in effluxes conducted at 17 degrees C or under ouabain. Its size decreased by microscopic dissection of the dense adventitia, which is probably its source.