ObjectiveTo determine the relative importance of the cardiac and vascular sympathetic components of the orthostatic response to 90° head-up tilt after N-type calcium-channel blockade in normotensive (sham renal cellophane wrap) and hypertensive (renal wrap) conscious rabbits.MethodsThe effects of N-type calcium-channel blockade with ω-conotoxin GVIA (ω-CTX, 10 μg/kg i.v. bolus) were assessed in the absence or presence of cardiac block by propranolol and methscopolamine. These were contrasted with the effects of α1-adrenoceptor antagonism (prazosin 0.5 mg/kg i.v. bolus, in the presence of cardiac block) or ganglion blockade (mecamylamine 4 mg/kg i.v. bolus).ResultsIn vehicle (0.9% saline) treatment groups, the response to tilt consisted of a small pressor effect (4 ± 2 and 7 ± 1 mmHg) and tachycardia (29 ± 6 and 17 ± 6 beats/min) in sham (n= 6) and wrap (n= 5) rabbits, respectively. After prazosin administration (with cardiac block), there were significant falls in MAP of 3 ± 1 and 7 ± 2 mmHg in sham (n= 7) and wrap (n= 6) rabbits, respectively, in response to tilt. ω-CTX caused postural hypotensive responses of 8 ± 2 and 13 ± 2 mmHg in sham (n= 6) and wrap (n= 7) rabbits, respectively, and 7 ± 1 and 14 ± 2 mmHg in sham (n= 7) and wrap (n= 7) rabbits with prior cardiac block. Similarly, mecamylamine caused falls in MAP of 8 ± 1 and 10 ± 2 mmHg in response to tilt in sham (n= 6) and wrap (n= 9) animals, respectively.ConclusionSympathetic vasoconstrictor effectors are primarily responsible for maintaining blood pressure during tilt in conscious rabbits. The postural hypotension caused by sympatholytic agents is about double in hypertensive rabbits, and N-type calcium-channel blockade is as effective as ganglion blockade at inducing this syndrome.