Pharmacological properties of presynaptic .BETA.-adrenoceptors in guinea-pig pulmonary arteries.

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Abstract
Pharmacological properties of the facilitatory presynaptic .beta.-adrenoceptor mechanism were studied in superfused spiral preparations of guinea pig pulmonary arteries preloaded with 3H-norepinephrine (.sbd.)-isoproterenol (0.3 .mu.M)-induced increases in total 3H efflux per pulse evoked by transmural field stimulation (1, 5, 10 and 20 Hz, 10 V, 2 ms pulse width, 100 pulses and 30 min intervals) were neither dependent on impulse-frequencies nor selective at lower frequencies. Isoproterenol increased 3H efflux at 5 Hz in a concentration-dependent manner (1 nM to 1 .mu.M): pD2 was 7.7. Salbutamol increased 3H efflux in a similar manner to isoproterenol: pD2 was 7.4. Prenalterol at 3 .mu.M only slightly increased 3H efflux. Trazolol (10 nM to 3 .mu.M) produced no increases. Atenolol (3 .mu.M) and practolol (3 .mu.M) did not antagonize isoproterenol (0.3 .mu.M)-induced increases in 3H efflux. Butoxamine (3 .mu.M) and H 35/25 (3 .mu.M) did antagonize this parameter. (.sbd.)-Epinephrine (1 nM to 0.1 .mu.M) decreased 3H efflux at 5 Hz and concentration-dependently increased this parameter in the presence of 10 .mu.M phentolamine. (.sbd.)-Norepinephrine (10 nM to 1 .mu.M) concentration-dependently inhibited evoked 3H efflux and did not increase the parameter in the presence of 10 .mu.M phentolamine, 10 .mu.M cocaine and 10 .mu.M normetanephrine. There exist presynaptic .beta.2-subtype receptors on noradrenergic nerve endings innervating guinea pig pulmonary arteries.