Hypoxic vasoconstriction of cyclostome systemic vessels: the antecedent ofhypoxic pulmonary vasoconstriction?

Hypoxic vasoconstriction (HV) is an intrinsic response of mammalian pulmona ry vascular smooth muscle (VSM). In the present study, HV was examined by m yography of vessel rings from three primitive vertebrates: New Zealand hagf ish (NZH), Pacific hagfish (PH), and sea lamprey (SL). Hypoxia dilated pre- gill arteries (ventral aorta, afferent branchial) from all species, whereas it contracted systemic arteries [dorsal aorta (DA), efferent branchial, ce liacomesenteric]. DA HV was reproducible over several days, and it could be sustained in NZH for 8 h without adverse effects. Tension was proportional to PO2, and half-maximal HV was obtained at PO2 (mmHg) of 4.7 +/- 0.2 (NZH ), 0.8 +/- 0.1 (PH), and 10.7 +/- 1.9 (SL). HV did not require precondition ing (preexisting contractile stimulus) and was unaffected by elevated extra cellular potassium (200 mM NZH; 80 mM SL); removal of the endothelium (NZH) ; or inhibitors of cyclooxygenase, lipoxygenase, cytochrome P-450 or antago nists of alpha -adrenergic, muscarinic, nicotinic, purinergic, or serotonin ergic receptors. These results show that HV is an intrinsic feature of syst emic VSM in cyclostomes and suggest that HV has been in the repertoire of V SM responses, since the origin of vertebrates. The exceptionally hardy HV i n cyclostome DA may provide a useful model with which to examine both the p hylogeny and mechanisms of this response.