THERMAL AND OSMOTIC RESPONSES OF ARTERIAL RECEPTORS

Carotid bodies (or carotid sinuses) and their own nerves were excised from anesthetized cats and superfused with physiological solutions of different compositions equilibrated with different gas mixtures. Unitary discharges were recorded from small nerve filaments and intracellular potentials were registered from carotid body (glomus) type I cells. Temperature of the bathing solution was varied between 30 and 40°C; and baro- and chemosensory discharge frequency increased as temperature was raised. Baroreceptor units showed a Q10 of 2.3 and a mean activation energy (μ) of 15.8 kcal/mol. Arterial stretch increased the sensory discharge frequency, but the activation energy did not change. Chemoreceptor discharges under basal conditions (50% O2 in N2) had a mean Q10 of 75, corresponding to a mean value of μ 81.5 kcal/mol (range, 32.5-144.8 kcal/mol). Receptor stimulation by air (about 20% O2 in N2) increased the frequency of sensory discharges although μ did not change. When receptors were made less active by bathing them in 100% O2 the discharge frequency decreased and the mean value of μ fell to 48% of its original value. Stimulation of chemoreceptors with 6% CO2 in 50% O2 and 44% N2 increased the sensory discharge frequency, but the energy of activation did not change if pH was maintained at control values (7.43). When pH was allowed to fall to 6.0-6.5, the discharge frequency increased above the levels obtained with CO2 at normal pH, but the energy of activation was less (41%) than in the control. When chemoreceptors were stimulated with a CO2-free acid solution (pH 6.0-7.0), the discharge increased but μ was smaller (52-55%) than in the control. When pH of the bathing solution was increased to 7.8, the discharge frequency of the receptor decreased but μ was 50% higher than at normal pH. Osmotic changes induced small but significant effects on baroreceptor discharges. A 10-40% increase in osmolality reduced their frequency of discharge, whereas hypoosmotic solutions -10 to -25%) had the opposite effect. Hyperosmotic solutions (+15 to +31%) increased the frequency of discharge of chemosensory fibers. Hypoosmotic solutions (-8 to -23%) had the opposite effect. Glomus type I cells were depolarized in hyperosmotic solutions (+5 to +45%) and their input resistance decreased accordingly. In hypoosmotic solutions (-15 to -22%), these cells became hyperpolarized and their input resistance increased considerably. Results are discussed in terms of some receptor mechanisms and possible influences on the central nervous system.