In the pulmonary vein (PV), two phenotypically distinct types of myocytes have been reported. Specifically, bundles of cardiac-like myocytes are found around typical spindle shaped smooth muscle cells, and are thought to be involved in modulating PV tone (Michelakis et al. 2001). Intrapulmonary veins contribute significantly to the total pulmonary vascular resistance and are known to constrict during hypoxia (Zhao et al. 1993). Despite this, the effect of hypoxia on [Ca²⁺]_i in the pulmonary vein has received little attention. Thus, we have examined [Ca²⁺]_i signalling in these two distinct myocyte phenotypes during hypoxia.

Male Sprague-Dawley rats (200-300g) were killed by cervical dislocation and the heart and lungs removed en bloc. Myocytes were then isolated from intrapulmonary veins (> 400 µm) using methods similar to those used for the pulmonary artery, as described by Drummond & Tuft (1999) and incubated with 5 µM fura-2 AM. Hypoxic solutions were achieved by bubbling extracellular bath solution with N₂. All experiments were carried out at room temperature. Where appropriate mean data ± S.E.M. are given and n is the number of cells studied. Statistical differences were tested for using Student’s paired t test. P < 0.05 was considered to be significant.

In cardiac-like myocytes isolated from the vein, spontaneous oscillations in [Ca²⁺]_i were observed. Under normoxic conditions (PO₂ ~145 mmHg), these oscillations had a frequency of 0.6 ± 0.2 Hz. During the oscillations, [Ca²⁺]_i increased from a basal level of 113 ± 17 nM to 333 ± 26 nM. Hypoxia (PO₂ ~20 mmHg) increased oscillation frequency to 1.1 ± 0.1 Hz (P < 0.05), and also increased basal [Ca²⁺]_i to 178 ± 14 nM (P < 0.01), in five cells. During hypoxia, the peak amplitude of the [Ca²⁺]_i oscillations increased to 553 ± 33 nM (P < 0.05). These effects were all reversible on return to normoxic conditions. Typical spindle-shaped smooth muscle cells did not show spontaneous oscillations. Hypoxia (PO₂ ~20 mm Hg) increased basal [Ca²⁺]_i from 105 ± 4 nM to 156 ± 3 nM in 3 of 10 cells. However, hypoxia increased the peak amplitude of caffeine-induced Ca²⁺ transients in these cells from 706 ± 35 nM during normoxia to 1630 ± 51 nM during hypoxia (n = 10, P < 0.01).

These results show that hypoxia affects [Ca²⁺]_i signalling in the two distinct phenotypes of myocytes found in the pulmonary vein.

This work is funded by the British Heart Foundation.