Vishnu can be called ishana by theological interpretation in vedas because he is the husband of Bhudevi (Aditi) .
- विष्टम्भो दिवो धरणः पृथिव्या अस्येशाना जगतो विष्णुपत्नी ।
- “Support of heaven, sustainer of the earth, lord /ruler of this universe, wife of Viṣṇu.”
Taittirīya Saṃhitā 4.4.12
īśānā jagato viṣṇupatnī.
He is identified as ishana directly in Mahabharata
- तस्मात्सर्वं सम्भवति जगत्स्थावरजङ्गमम् । सोऽनिरुद्धः स ईशानो व्यक्तः स सर्वकर्मसु ॥
- "From Him arises the entire universe, everything stationary and moving. He is Aniruddha; He is Īśāna; He manifests in all actions."
Ishana is also referred to Indra ,Soma and Varuna in Rigveda .
मुषाय सूर्यं कवे चक्रमीशान ओजसा । वह शुष्णाय वधं कुत्सं वातस्याश्वैः ॥ ४ ॥
This title isn't only applicable to a single diety as far as vedas are concerned. Rudra is more often identified as Ishana in the later portions of Brahmanas . Amara kosha which is composed in 4nd CE refers to the names of Shiva where he is invoked as following :
Amarakośa 1.1.30–31
ईश्वरः शर्व ईशानः
This comes more explicit in puranas